A test aimed at determining protein C in the blood for diagnosis possible reasons development of thrombosis and thrombotic complications.
Synonyms Russian
Protein C; PS; coagulation protein C.
SynonymsEnglish
Protein C; PC; coagulation protein C.
Research method
Kinetic colorimetric method.
What biomaterial can be used for research?
Venous blood.
How to properly prepare for research?
- Eliminate fatty foods from your diet for 24 hours before the test.
- Avoid physical and emotional stress for 30 minutes before the test.
- Do not smoke for 30 minutes before the test.
General information about the study
Protein C is one of the most important proteins - factors of the anticoagulant (anti-coagulation) system of the blood. The synthesis of this protein occurs in the liver and is vitamin K dependent. Protein C is in constant circulation in the blood in an inactive state. Its activation occurs when a complex of thrombin and thrombomodulin acts on the surface of intact endothelial cells and platelets. In its active form, protein C partially destroys and inactivates non-enzymatic coagulation factors Va and VIIIa. The enzymatic action of protein C occurs in the presence of its cofactor, protein S. It is a vitamin K-dependent non-enzymatic cofactor synthesized in the liver and circulating in the bloodstream. As a result of the described interactions, blood coagulation processes are inhibited, and the processes of the anticoagulation system (fibrinolysis) are also indirectly activated.
Determining the concentration or activity of protein C in the blood is important in the diagnosis of various pathological conditions and diseases. A decrease in these indicators may be associated with a violation of the synthesis of protein C, its rapid consumption, or a violation of the protein structure and its functional inferiority. Protein C synthesis can be reduced as a result of congenital deficiency, vitamin K deficiency, liver pathologies, disruption of its synthetic function, during the neonatal period and in the elderly. Excessive protein consumption can be observed in thrombosis, thromboembolism, consumption coagulopathies, disseminated intravascular coagulation syndrome (DIC), after major operations and injuries. Impairment of the functional activity of protein C can be observed when taking anticoagulant drugs, in particular when taking oral warfarin. An increase in the concentration of protein C can be observed during pregnancy, when taking estrogen-based oral contraceptives, and with kidney disease.
Congenital protein C deficiency occurs in 0.2-0.5% of cases and is characterized by a severe course. It requires preventive and therapeutic measures to prevent the development of thrombosis and fatal complications. A rare variant of homozygous protein C deficiency manifests itself as fulminant DIC syndrome in newborns and requires urgent diagnostic measures and treatment.
In pregnant women, protein C deficiency leads to a number of severe pathological processes and complications. Thrombosis and thromboemoliia may develop with damage to the deep veins of the lower extremities, pelvic organs, and cerebral vessels, and a possible complication in the form of pulmonary embolism. Intrauterine growth retardation as a result of fetoplacental insufficiency, spontaneous abortions and repeated miscarriages may occur. The risk of developing preeclampsia, eclampsia and disseminated intravascular coagulation increases.
When taking indirect anticoagulants and with a significant decrease in protein C activity to or less than 50% of the norm, skin necrosis may develop. Such “warfarin necrosis” develops rarely, but is characterized by a severe course and requires careful medical supervision. Therefore, it is recommended to carry out treatment with indirect anticoagulants under the control of protein C activity. Control and repeated determinations of protein C should be carried out at least a month after discontinuation of the drugs.
The main manifestations of protein C deficiency are arterial and venous thrombosis of various locations. Myocardial infarction, stroke, pulmonary embolism may occur in the absence of other predisposing factors and in persons young. Determination of the concentration/activity of protein C can also be recommended for oncological diseases, purulent-inflammatory diseases, sepsis and septic processes.
What is the research used for?
- To diagnose the concentration or activity of protein C;
- To diagnose the concentration or activity of protein C when identifying the causes of thrombophilias and thrombotic complications;
- To identify possible causes of arterial and venous thrombosis of various locations, in particular in young people;
- To diagnose the causes of thrombotic complications during pregnancy;
- To diagnose possible causes of the development of thrombotic complications in newborns, in the complex diagnosis of congenital protein C deficiency;
- For the diagnosis of protein C during treatment with indirect anticoagulants, warfarin;
- For the diagnosis of protein C in oncological, purulent-inflammatory diseases, sepsis.
When is the study scheduled?
- At comprehensive examination to identify the causes of thrombosis (determination of antithrombin III, protein S, etc.);
- At clinical manifestations arterial and venous thrombosis: myocardial infarction, stroke, pulmonary embolism, deep vein thrombosis of the lower extremities, pelvic organs, etc.;
- For symptoms of congenital thrombosis, presumably associated with protein C deficiency;
- For pregnancy pathologies: preeclampsia, eclampsia, disseminated intravascular coagulation syndrome, intrauterine growth retardation, spontaneous abortions, repeated miscarriages;
- When treated with indirect anticoagulants, warfarin; with the development of warfarin skin necrosis;
- With vitamin K deficiency, liver pathologies;
- For oncological, purulent-inflammatory diseases, sepsis.
What do the results mean?
Reference values
|
Age |
Reference values |
|
28 days – 3.5 months. |
|
|
6 months – 1 year |
|
|
More than 16 years |
Reasons for increased protein C levels:
- Pregnancy;
- Taking estrogen drugs;
- Kidney diseases.
Reasons for low protein C levels:
- Congenital protein C deficiency;
- Vitamin K deficiency;
- Liver pathologies;
- Thrombosis, thromboembolism;
- Disseminated intravascular coagulation syndrome (DIC syndrome);
- Extensive surgical operations, injuries;
- Taking anticoagulant drugs, in particular warfarin;
- Purulent-inflammatory diseases;
- Sepsis;
- Oncological diseases.
What can influence the result?
Taking indirect anticoagulant drugs, warfarin.
Important Notes
- Determining the level of protein C is recommended to be carried out along with comprehensive laboratory diagnostics of other indicators of the blood coagulation and anti-coagulation systems.
- It is recommended to carry out treatment with indirect anticoagulants under the control of protein C activity. Control and repeated determinations of protein C must be carried out at least a month after discontinuation of the drugs.
- Protein S free
- Antithrombin III
- Lupus anticoagulant
- Coagulogram No. 1 (prothrombin (according to Quick), INR)
- Thrombin time
- Coagulogram No. 2 (prothrombin (according to Quick), INR, fibrinogen)
- Coagulogram No. 3 (prothrombin (according to Quick), INR, fibrinogen, ATIII, APTT, D-dimer)
- Annexin V IgG antibodies
Who orders the study?
Therapist, general practitioner, hematologist, gynecologist, neonatologist, pediatrician, obstetrician-gynecologist, surgeon, anesthesiologist-resuscitator.
Literature
- Dolgov V.V., Menshikov V.V. Clinical laboratory diagnostics: national guidelines. – T. I. – M.: GEOTAR-Media, 2012. – 928 p.
- Fauci, Braunwald, Kasper, Hauser, Longo, Jameson, Loscalzo Harrison’s principles of internal medicine, 17th edition, 2009.
- Christiaans SC, Wagener BM, Esmon CT, Pittet JF. Protein C and acute inflammation: a clinical and biological perspective / Am J Physiol Lung Cell Mol Physiol. 2013 Oct 1;305(7):L455-66.
- Bouwens EA1, Stavenuiter F, Mosnier LO. Mechanisms of anticoagulant and cytoprotective actions of the protein C pathway / J Thromb Haemost. 2013 Jun;11 Suppl 1:242-53.
Determination method
Automatic analyzer of parameters of the coagulation system ACL TOP, method - kinetic colorimetric.
Material under study Plasma (citrate)
One of the most important natural clotting inhibitors.
Protein C is one of the most important physiological inhibitors of coagulation. In its active form, it cleaves and inactivates coagulation factors VIIIa and Va (but not factor V Leiden). Protein C exhibits anticoagulant activity, indirectly activates fibrinolysis, and limits the size of the blood clot. In vivo, protein C is activated by thrombin, many times accelerated by the complex of thrombin and thrombomodulin (a protein on the surface of endothelial cells).
The anticoagulant activity of protein C is enhanced by its cofactor -. Protein C is synthesized in the liver and is a vitamin K-dependent protein, so its activity also depends on vitamin K deficiency and oral anticoagulant therapy. Protein C levels in newborns and children younger age physiologically lower than in adults due to liver immaturity. Congenital protein C deficiency is associated with a tendency to severe thrombotic disorders. Among innate species deficiency of physiological anticoagulants, such as antithrombin III deficiency, protein C deficiency, protein S deficiency - protein C deficiency is the most common (0.2-0.4% of the population). Homozygous states appear in early childhood purpura fulminans of the newborn and is often fatal, protein C levels in these newborns are undetectable.
Patients with protein C deficiency are usually heterozygotes in whom thrombosis does not appear until the second or third decade of life. Among them, about 5% may also have a factor V mutation (factor V Leiden) in a heterozygous state. The presence of this mutation is considered a risk factor for the development of early thrombotic pathology (see genetic studies, thrombophilia, test No. 7171). Protein C deficiency is associated with an increased risk of pregnancy complications (deep vein thrombosis, preeclampsia, intrauterine growth restriction, and recurrent miscarriages). There is an increased risk of developing warfarin-induced skin necrosis. The effect of risk factors associated with bad habits is aggravated.
Congenital deficiency conditions can be diagnosed when causes of acquired protein C deficiency have been ruled out. Protein C testing for this purpose is not recommended during acute illness/acute thrombotic episodes due to protein C consumption, or in patients receiving oral anticoagulant therapy (warfarin reduces protein C levels).
Repeated testing of protein C is recommended after cessation of oral coagulant therapy (preferably one month after the end of therapy), in correlation with examination of family members. In heterozygotes for protein C deficiency, values partially overlap the normal reference range. Impaired activation of protein C occurs in pathological conditions associated with the presence of factors such as hypoxia, endotoxin, interleukin-1, tumor necrosis factor alpha, high levels of homocysteine (all of which accelerate coagulation by inducing the expression of tissue factor and suppressing the transcription of thrombomodulin by endothelial cells).
The information value of testing protein C for prognostic purposes in septic conditions (characterized by increased consumption, destruction and impaired synthesis of protein C) is shown. Protein C activity level< 40%, а также снижение более чем на 10% за 1 день при сепсисе коррелирует с неблагоприятным прогнозом.
Literature
Shorr A.F. R92 Protein C concentrations in severe sepsis: an early directional change in plasma levels predicts outcome Critical Care 2006,10: R92 http://ccforum.com/content/10/3/R9.
Methodological materials reagent manufacturer.
Protein C is a protein with anticoagulant activity, one of the main factors of the anticoagulant system that maintains blood in a liquid state. This indicator has an independent diagnostic value, but is more often used in conjunction with the determination of protein S in the bloodstream. The main indications for testing are thrombosis or suspicion of hereditary thrombophilia. Plasma isolated from venous blood is used for analysis. Most often, the study is carried out using the kinetic colorimetric method. Range of standard indicators for adults: activity – from 70 to 140%; concentration from 2 to 6 mg/l. Depending on the laboratory and method, the turnaround time for analysis ranges from 1 to 14 days.
Protein C is a protein with anticoagulant activity, one of the main factors of the anticoagulant system that maintains blood in a fluid state. This indicator has an independent diagnostic value, but is more often used in conjunction with the determination of protein S in the bloodstream. The main indications for testing are thrombosis or suspicion of hereditary thrombophilia. Plasma isolated from venous blood is used for analysis. Most often, the study is carried out using the kinetic colorimetric method. Range of standard indicators for adults: activity – from 70 to 140%; concentration from 2 to 6 mg/l. Depending on the laboratory and method, the turnaround time for analysis ranges from 1 to 14 days.
Protein C is a physiological clotting inhibitor. In the active phase, it can cleave and inactivate coagulation factors VIIIa and Va. Protein C is an anticoagulant, therefore it promotes active fibrinolysis and reduces blood clots in size. Intracellularly, protein C can be activated only by thrombin or thrombin in combination with thrombomodulin. The anticoagulant effect of protein C is enhanced by a cofactor, protein S. Protein C is synthesized in the liver (hepatocytes). The anticoagulant is considered a vitamin K-dependent protein, so its activity varies depending on the concentration of vitamin K and heparin therapy. Due to the functional immaturity of the liver, the amount of protein C in the blood of newborns and infants up to one year is lower than the reference values for adults.
There are several inherited disorders in the synthesis of physiological anticoagulants. Compared to antithrombin III deficiency and protein S deficiency, protein C deficiency is the most common (about 0.3% in the population). Genetically inherited deficiency causes serious thrombotic pathologies. Homozygous changes can cause the appearance of purpura fulminans of the newborn in infancy, which in most cases has fatal outcome. The concentration of protein C in sick children is close to zero.
Tests for the determination of protein C play an important diagnostic and prognostic value in obstetrics, since thanks to the test it is possible to identify dangerous disorders during pregnancy. For example, with antiphospholipid syndrome, antibodies to other components of the anticoagulant system (protein C, thrombomodulin and protein S) are formed in the blood. This syndrome is very dangerous for the fetus, as it can cause spontaneous abortion or premature birth. Testing for protein C is also widely used in gynecology, as it helps diagnose ovarian hyperstimulation syndrome in women or predict unsuccessful IVF attempts that occur when hemostasis is impaired. In surgery, the test is used before surgery to detect and calculate the risk of bleeding.
Indications and contraindications
The study is prescribed if hereditary thrombophilia is suspected, especially if there are relatives in the family who suffer from this pathology. For prognostic purposes, the analysis may be indicated to assess the risk of developing thrombosis or thromboembolism before taking hormonal contraceptives. The analysis is also used for the differential diagnosis of coagulation system disorders (for example, in liver disease or in the postoperative period). The study is prescribed as part of pregnancy planning or in cases of recurrent miscarriage, as well as before starting therapy with indirect anticoagulants.
Relative contraindications to testing protein C levels are considered to be the acute phase of an infectious disease, sepsis or acute thrombosis. It is also not recommended to perform the test while taking oral contraceptives or anticoagulants, since the results will not be reliable (warfarin reduces the activity of protein C). In this case, you need to take a break from treatment, during which a study should be carried out.
Preparation for analysis and collection of material
The test uses plasma isolated from venous blood. It is placed in a “citrate” tube and, if necessary, transported in a special box to the laboratory. Before collecting biomaterial, the laboratory technician must ask the patient about taking medications that may affect the results of the analysis. It is recommended to conduct the study in the morning, since there are circadian rhythms at which biochemical parameters change. It is believed that reference standards reflect statistical results when blood is taken in the first half of the day.
The patient is also advised to avoid fatty foods and sugary drinks. You can only drink still water. If possible, physical and emotional stress that contributes to the occurrence of biochemical changes should be avoided. It is prohibited to drink alcohol and smoke 2-3 hours before taking the test. Physiotherapeutic and instrumental procedures cause temporary changes in laboratory parameters, so it is important for the patient to donate blood in advance to determine protein C.
The colorimetric kinetic research method is considered the most common. With the kinetic method, 2 control plasmas are used for internal quality control: one with normal parameters, and the other with pathological ones. This method consists of measuring the absorption of monochrome light (usually wavelength 540 nm). When light passes through the cuvette, a chromophore formation reaction occurs. The absorption rate is directly proportional to the level of protein C in the test plasma. The analysis period is 1 working day (may extend to 7-14 days depending on the workload of the laboratory).
Normal values
Protein C can be measured in two units: activity is determined as a percentage (%) and concentration in mg/l (milligrams per liter). In infants up to one year old and newborns, protein C activity is lower due to insufficient synthesis of the anticoagulant in the liver, which is considered a normal variant. Reference values for protein C concentrations in adults range from 2 to 6 mg/L.
Anticoagulant activity depends on age:
- newborns (1 day) – 26-44%;
- newborns (day 5) – 31-53%;
- newborns (30 days) – 32-54%;
- infants (3 months) – 41-67%;
- infants (6 months) – 48-70%;
- children from 1 year and adults – 70-140%.
Increased blood levels
The main reason for the increase in the concentration of protein C in the blood is the use of oral contraceptives, due to which the balance of the coagulation and anticoagulation systems is disturbed. Another reason for the increase in the concentration of protein C in the blood is the period of gestation. If a pregnant woman has previously been diagnosed with thrombosis of the veins of the lower extremities, the doctor should give a referral for a study, although it is not included in the screening program. Usually increased level anticoagulant does not carry important diagnostic value.
Decreased blood levels
The main reason for the decrease in the concentration of protein C in the blood is structural abnormalities of the coagulation factor V gene. The hereditary form of thrombophilia manifests itself in the patient from birth. Acquired anticoagulant deficiency can be acute or chronic, temporary or long-term. Occurs with liver disease or insufficient production of vitamin K (hepatitis, cirrhosis). In some cases in adults, acquired anticoagulant deficiency does not lead to thrombosis, since the concentration of coagulation factors also decreases. IN childhood acquired protein C deficiency can occur due to the addition of a bacterial infection (for example, with meningitis), when toxins increase the risk of blood clots.
Treatment with warfarin may also cause a decrease in the concentration of protein C in the blood. The concentration of protein C is always reduced in patients to whom it was prescribed, so during therapy it is not advisable to conduct a study to determine the level of this physiological inhibitor of coagulation. If it is necessary to control anticoagulant therapy, warfarin is discontinued 2 weeks before the analysis. If during the period of discontinuation of warfarin there is a risk of exacerbation of thrombosis, the doctor may prescribe a low molecular weight heparin drug.
Treatment of abnormalities
An analysis to determine the concentration of protein C is important in clinical practice, especially for patients with thrombosis or hereditary diseases associated with disruption of the anticoagulant system. To quickly correct physiological deviations from the norm, it is important to increase immunity, follow a diet, normalize the drinking regime, move more and actively engage in sports. When you receive the result of an analysis to determine the activity of protein C, you should immediately contact your doctor: phlebologist, gynecologist, surgeon, hepatologist, nephrologist, infectious disease specialist. In order to quickly normalize the patient's condition, the doctor may prescribe injections of sodium heparin or warfarin.
For diseases of the kidneys, liver, endocrine system, and infectious processes, determination of blood protein is indicated. This analysis is part of a biochemical study. C-reactive protein is an indicator of the activity of the inflammatory process, used in diagnosis, determining the effectiveness of therapy, and the risk of cardiovascular diseases. C and S proteins reflect the anticoagulation system of the blood.
Read in this article
Explanation of terms
A blood test for protein content is part of a biochemical study. It is prescribed for many diseases. To understand the main names that appear in the results, you need to know the meaning of some terms:
- protein total, protein total– the sum of the content of albumins and globulins, the total concentration of proteins;
- C-reactive protein (pronounced "c")– indicator of the intensity of the inflammatory process;
- C-protein– inhibits the formation of blood clots, has an effect opposite to coagulation factors (proteins that thicken blood);
- S protein– enhances the activity of C-protein;
- blood for eosinophilic cationic protein studied in allergic diseases to diagnose and determine the severity.
Blood protein analysis
Plasma protein is represented mainly by albumins and globulins. The former are formed by the liver and make up about 60% of the total blood protein. Globulins, in addition to the liver, are produced by cells immune system. A referral for analysis can be issued by a surgeon, cardiologist and nephrologist. Total blood protein is part of a standard biochemical test.
Indications and performance
Pathological conditions in which the content of proteins in the blood may have diagnostic value, are:
- frequent subcutaneous hemorrhages, tendency to bleed;
- the appearance of blood in the stool;
- decreased urine output, swelling of the legs, pain in the lower back, bones;
- starvation, emaciation;
- suspicion of oncological and autoimmune processes;
- insufficiency of liver and kidney function;
- infectious diseases;
- burns.
Blood test from a vein The analysis requires venous blood taken after a 10-hour break in food intake. On the morning of the test, you can drink exclusively clean water.
Normal when blood levels are low and high
The results of the analysis for blood protein content are compared with tables that indicate values corresponding to age. For example, for newborns the norm is considered to be from 45 to 67 g/l, and for adults – 64 to 84 g/l. Indicators are elevated when:
- dehydration (severe diarrhea or vomiting, extensive thermal burn, ketoacidosis in diabetes, hyperosmolar coma);
- acute and chronic infections;
- connective tissue diseases (lupus erythematosus, scleroderma, rheumatism);
- multiple myeloma.
A decrease in the concentration of proteins in the blood may be due to the following reasons:
- lack of intake during fasting, acute pancreatitis, intestinal diseases, parenteral nutrition;
- liver damage – cirrhosis, tumor, hepatitis, poisoning;
- loss due to bleeding, nephritis, renal amyloidosis, nephropathy, burns;
- protein breakdown during prolonged fever, injuries, thyrotoxicosis, excessive physical activity, oncology;
- copious administration of solutions during infusion therapy;
- long-term use of steroid hormones, anabolic steroids;
- transfer of blood plasma into the pleural, abdominal or pericardial cavity with significant effusion (exudation).
Blood consists not only of well-known cells - erythrocytes, leukocytes, but also of various organic compounds, related in their structure to protein substances When is a protein C and protein S test needed?
One of the main factors in the body's anticoagulant system is protein C. It prevents the formation of a blood clot. Its cofactor, protein S, is also of diagnostic importance. This protein enhances the effect of protein C and ensures the maintenance of blood fluidity. They are formed in the liver, their synthesis depends on the content of vitamin K in the body and the use of drugs that act on blood clots.
Indications for the diagnosis of these indicators are:
- frequent venous or arterial thrombosis in young patients;
- miscarriage;
- carrying out treatment with anticoagulants (before starting use);
- postoperative period;
- suspicion of hereditary thrombophilia;
- liver diseases.
Normal levels of protein C for children under 1 year of age and adults are from 70 to 140 percent, and protein S levels are 20 to 25 mg/l. The study of these indicators is usually carried out during a coagulogram.
A decrease in these values occurs when:
- congenital deficiency ();
- liver dysfunction;
- high consumption in case of intravascular coagulation;
- infections, including HIV;
- kidney diseases;
- malignant neoplasms.
C-protein is always reduced during use. An increase in the level of this protein has no diagnostic value.
C-reactive protein is a marker of inflammation
After the penetration of a foreign protein (virus, bacteria), the formation of C-reactive protein increases in the liver. In the first days it can exceed the norm by tens and even hundreds of times. This protein can be “good” (increases immune defense) and “bad” (disturbs the condition of the inner lining of blood vessels, stimulates spasm,).
Indications for analysis:
For all age categories, the concentration of this protein should not exceed 5 mg/l. During pregnancy, an increase to 20 mg/l is allowed. For pathological conditions, it is important to take into account dynamic changes in the indicator, since they reflect the deterioration or improvement of the patient’s condition. High values of C-reactive protein occur with:
- diseases of the digestive system;
- acute and chronic inflammation;
- rheumatism and autoimmune diseases;
- amyloidosis;
- transplant rejection;
- malignant neoplasms and metastases;
- viral and bacterial infections;
- septic process;
- deep burns;
- after operations;
- tuberculosis;
- meningitis.
D.H. KHIZROEVA, I.A. MIKHAILIDI, N.S. STULEV
First Moscow State University named after I.M. Sechenova, 119991, Moscow, st. Trubetskaya, 8, building 2
Khizroeva Jamilya Khizrievna 1
Stuleva Nadezhda Sergeevna- Candidate of Medical Sciences, assistant at the Department of Obstetrics and Gynecology, Faculty of Preventive Medicine, tel. +7-915-361-90-73, e-mail: 1
Mikhailidi Irina Arkhimedovna- postgraduate student of the Department of Obstetrics and Gynecology, Faculty of Preventive Medicine, tel. +7-903-798-08-97, e-mail: 1
Activated protein C (APC) interacts with the endothelial protein C receptor (EPCR), protease-activated receptors (PAR), apolipoprotin E2 receptor and integrins, has various effects on the hemostasis system (anticoagulant effect) and the body's immune system (cytoprotective effect). The importance of the protein C system is best demonstrated by the prothrombotic and inflammatory complications caused by protein C deficiency or dysfunction, which in clinical practice manifest as ischemic stroke, inflammatory disease, atherosclerosis, vascular complications and obstetric problems. Studying and understanding the biological function of APC makes it possible to control coagulation and inflammation and find the use of protein C preparations as an anticoagulant and cytoprotector in the clinical practice of a doctor.
Keywords:activated protein C, endothelial protein C receptor, factor mutationVLeiden, APC resistance, thrombosis.
D.Kh. KHIZROEVA, I.A. MIKHAYLIDI, N.S. STULEVA
I.M. Sechenov First Moscow State Medical University, 8-2 Trubetskaya St. , Moscow, Russian Federation 119991
Significance of protein C determination in obstetric practice
Khizroeva D.Kh.- Candidate of Medical Science, assistant of the Department of Obstetrics and Gynecology of Public Health Faculty, +7-915-361-90-73, e-mail:
Stuleva N.S.-Candidate of Medical Science, assistant of the Department of Obstetrics and Gynecology of Public Health Faculty, +7-915-361-90-73, e-mail: 1
Mikhaylidi I.A.- postgraduate student of the Department of Obstetrics and Gynecology of Public Health Faculty, +7-915-361-90-73, e-mail: 1
Activated protein C (APC), interacting with the endothelial protein C receptor (EPCR), receptors, activated by proteases (PAR), apolipoprotina E2 receptor and integrins, has various effects on the hemostatic system (anticoagulant effect) and the immune system (cytoprotective effect).Value of protein C is best demonstrated with prothrombotic and inflammatory complications caused by protein C deficiency or violation of its functions, which in clinical practice appear as ischemic stroke, inflammatory disease, atherosclerosis, vascular complications and obstetric problems.Learning and understanding the biological function of APC provides control over coagulation and inflammation and understanding the use of drugs with protein C as anticoagulant and cytoprotector in clinical practice of a physician.
Key words:activated proteinC, endothelial protein C receptor, factor V Leiden mutation,ARS resistance, thromboses.
The introduction of knowledge on theoretical and clinical hemostasiology into clinical practice has significantly deepened our understanding of the pathogenesis of various complications in obstetric practice. One of the important components of the hemostatic system that regulates both hemostasis and the human immune system in response to vascular or inflammatory injury is the protein C system.
The protein C system is the body's natural anticoagulant system, regulating clotting, maintaining blood fluidity, preventing thrombosis, thus preventing vascular damage and stress. The key protease of the protein C system is activated protein C (APC). Protein C was first isolated in 1975 by Dr. Johan Stenflo, professor at the Department of Clinical Biochemistry at Lund University (Sweden). Before this, in 1960, protein C was discovered by Professor Seegers, who gave the first name to protein C - autoprothrombin IIa, or coagulation factor XIV. Later, Professor Stenflo, studying the prothrombin profile, isolated several substances using chromatography and the third protein (peak C) was a new vitamin K-dependent protein, which, accordingly, was named protein C. Stenflo continued further study of protein C in the laboratory John Suttie in Madison, Wisconsin, where he worked with postdoctoral fellow Charles Esmon, who converted the original inactive protein C into its active form by proteolysis with trypsin (but not thrombin or factor Xa). Further attempts to determine the properties of the new protein and its role in the reactions of coagulation and platelet aggregation led a group of scientists from Seattle (Kisiel, Ericsson and Davie) to the conclusion that trypsin-activated protein C does not increase thrombin formation or platelet aggregation, but, on the contrary, exhibits a rather noticeable anticoagulant effect.
In addition to its anticoagulant activity, activated protein C has cytoprotective and anti-inflammatory effects on vascular endothelial cells, neuronal cells and various cells of the human immune system. These pleiotropic effects of the protein C system on the hemostasis and inflammation systems gave impetus to new research and led to the creation of recombinant APC, which has found use in the treatment of severe sepsis (PROWESStrial).
The human protein C gene is encoded on chromosome 2. Protein C (a glycoprotein with a molecular weight of 62,000 daltons, the precursor of a serine protease) is synthesized as a single polypeptide chain containing a light chain with a molecular weight of 21,000 daltons and a heavy chain with a molecular weight of 41,000 daltons, connected by a disulfide bond. In terms of amino acid sequence and structure, it is highly homologous with thrombin and other vitamin K-dependent coagulation factors - FVII, FIX, FX. Its minimum concentration in the blood plasma of healthy people is approximately 3 mg/ml, which is equivalent to 60 nmol/l.
Protein C is synthesized in the liver and consists of light and heavy chains, molecular weight - 62000 Da. Physiological proteolytic activation of protein C by thrombin occurs on the surface of endothelial cells with the participation of trypsin and two membrane receptors, thrombomodulin and endothelial protein C receptor (EPCR). Thrombomodulin is a high-affinity thrombin receptor. Thrombin associated with thrombomodulin, as a result of a change in the conformation of the active center, changes the direction of its action. Thrombin becomes more sensitive to inactivation by antithrombin III and completely loses its ability to interact with fibrinogen and activate platelets. When complexed with thrombin, thrombomodulin functions as a cofactor to accelerate the activation of protein C. EPCR is a key protein C receptor in regulating the various actions of activated protein C (APC). The binding of thrombin to thrombomodulin promotes the activation of protein C. This reaction is enhanced when protein C is localized on the endothelial surface in conjunction with EPCR (Fig. 1). For example, activation of protein C by the thrombin-thrombomodulin complex is 1000 times higher than activation by thrombin alone in the absence of TM, and it is enhanced 10-20 times more if protein C is coupled to its EPCR receptor.
Figure 1.
Components and effects of the Protein C system. The three major reactions of Protein C, depicted from left to right, are Protein C activation, the Protein C anticoagulant pathway, and the Protein C cytoprotective pathway. On the left is Protein C activation. Physiological activation of Protein C (Pc) by the thrombin complex (Iia)-thrombomodulin (TM) on the surface of endothelial cells is promoted by EPCR. In the middle is the anticoagulant pathway of protein C. APC exerts its anticoagulant effects by proteolytic inactivation of Fva and Fviiia, with the help of Ps on negatively charged phospholipid membranes. On the right is the cytoprotective pathway of protein C. APC coupled to EPCR cleaves Par1 to initiate intracellular signaling pathways with the development of cytoprotective effects, which include anti-inflammatory and anti-apoptotic activities, disruption of gene expression profiles and barrier protective actions.
Dissociation of APC from EPCR leads to its release into the plasma, where APC is inactivated by plasma serine protease inhibitors (serpins), including proteinase a1 inhibitor (a1-PI), plasminogen activator inhibitor I (PAI-I), protein C inhibitor (PCI), etc. PCI-mediated inhibition of APC is enhanced by heparin, while the formation of the APC-PAI-I complex is accelerated by vitronectin. PCI may also inhibit the binding of thrombin to TM. The discovery of the protein C inhibitor PCI in areas of brain damage in patients with sclerosis led scientists to study the potential effectiveness of APC in mice for sclerosis and sclerosis-like diseases.
Neutralization of PAI-I in combination with APC increases fibrinolytic potential. Therapeutic administration of high doses of APC is associated with stimulation of fibrinolysis. Another mechanism for enhancing the fibrinolysis process by activated protein C is associated with the anticoagulant effect of APC on thrombin formation, which leads to a decrease in the activation of TAFI (thrombin-activated fibrinolysis inhibitor) by the thrombin-TM complex.
Anticoagulant activity of protein C
As an anticoagulant enzyme, APC inactivates factors Va (FVa) and VIIIa (FVIIIa) by proteolytic proteolysis. Circulating inactive factor V has the potential to exhibit procoagulant or anticoagulant activity depending on modification by pro- or anticoagulant enzymes. Under the influence of thrombin, active factor V is formed, which has procoagulant activity. After proteolytic inactivation by activated protein C, FVa is converted into the inactive factor FVi. Cleavage of FVa by activated protein C begins at the Arg 506 site, after which FVa loses its ability to interact with FXa. Complete inactivation of FVa occurs after cleavage at Arg 306. Since FVa enhances prothrombinase production ~10,000-fold, inactivation of FVa by APC effectively reduces thrombin formation. Inactivation of factors FVa and FVIIIa on negatively charged phospholipid membranes is carried out with the help of cofactors - protein S and factor V (Fvac). The importance of protein S is confirmed by the fact that its deficiency in human blood is accompanied by thromboembolic complications. FVac is formed by activation of factor V by activated protein C and has anticoagulant activity. In this case, a cofactor of activated protein C is also formed, which participates together with protein S in the inactivation of FVIIIa. For the occurrence of APS cofactor activity, cleavage at the Arg 506 site is also fundamentally important. Factor FVac is converted to inactive factor FVi under the influence of thrombin.
Accordingly, FV, FVIIIa is an important cofactor for the tinase complex, which enhances the formation of factor Xa (FXa) approximately 200,000-fold. According to FVa, inactivation of FVIIIa by APC occurs after cleavage at Arg336 and Arg562. Unlike FVa, cleavage of FVIIIa at any site results in complete loss of activity. PS and FVac, but not FVa, enhance APC-mediated inactivation of FVIIIa.
Cytoprotective properties of APC due to its ability to inhibit the expression of proinflammatory cytokines, adhesion molecules, and prevent leukocyte adhesion. The functions of activated protein C (APC), as a modulator of inflammation, are realized through its receptors - the endothelial receptor EPCR and protease-activated receptor 1 (PAR1) on endothelial cells, monocytes and other cells. APC inhibits apoptosis and blocks inflammation, changing the gene expression profile in endothelial cells, reduces the production of pro-inflammatory cytokines by activated monocytes, and protects the endothelial barrier function. APC induces protective genes by activating either EPCR or the EPCR-PAR1 receptor cascade. APC-mediated cytoprotective signaling requires colocalization of PAR1 and EPCR in caveolin-1-enriched lipid aggregates or caveolae, possibly resulting from EPCR occupancy and initiated when EPCR-bound APC activates PAR1. In addition to many studies suggesting that PAR1 and EPCR are required to mediate the protective effects of APC on cells, other receptors such as sphingosine-1-phosphate receptor 1 (S1P1), apolipoprotein E receptor 2 (ApoER2), glycoprotein Ib, CD11b/CD18 (αMβ2; Mac-1; CR3), PAR-3 and Tie2 may also, individually or collectively, promote APC-initiated signaling on endothelial and other cells. About 20 genes are known whose expression is increased by APC and 20 genes whose expression is suppressed by APC. The former include genes with anti-inflammatory and anti-apoptotic activity, the latter - with pro-inflammatory and pro-apoptotic activity. APC has an anti-inflammatory effect on endothelial cells and leukocytes. The effect on endothelial cells is carried out by inhibiting the release of pro-inflammatory mediators and reducing vascular endothelial adhesion molecules. This reduces the adhesion of leukocytes, infiltration in tissues and limits the focus of destruction of the underlying tissues. APC supports endothelial barrier function and reduces chemotaxis. APC inhibits the release of inflammatory response mediators in leukocytes as well as in endothelial cells, reducing the cytokine response and reducing the systemic inflammatory response, as seen in sepsis. ARS has a neuroprotective effect. The anti-apoptotic effect of APC was the reason for prescribing recombinant APC drugs in the treatment regimen for sepsis, since a decrease in the degree of apoptosis correlated with higher survival of septic patients. APC protects the endothelial barrier. It is known that disruption of the endothelial barrier and a corresponding increase in endothelial permeability is associated with edema, hypotension, and inflammation that accompany sepsis.
The importance of the protein C system is best illustrated by the prothrombotic and proinflammatory complications caused by protein C deficiency or dysfunction in conditions such as ischemic stroke, inflammatory diseases, atherosclerosis, obstetric problems, etc. Protein C deficiency can be genetic or acquired.
Hereditary deficiency of protein C is autosomal dominant and increases the risk of thrombosis, the degree of which depends on homozygous or heterozygous carriage of the mutation. Currently, about 200 different mutations of the protein C gene are known. Some of them lead to almost complete loss of gene function and the development of neonatal purpura fulminantum, others have a slight effect on the function of the protein and slightly increase the risk of developing thrombophilia. The expression of protein C gene mutations appears to be largely dependent on the presence of other, including hereditary, risk factors, since the same mutations in different families can increase the risk of thrombus formation in a heterozygous or only homozygous state. Homozygous carriage of protein C deficiency is quite rare and contributes to the development of neonatal purpura fulminantum or disseminated intravascular coagulation syndrome in infancy. WITH high level mortality in the absence of protein C replacement therapy. Heterozygous carriers are prone to venous thromboembolism. In addition, in individuals with a heterozygote, warfarin can cause a similar phenomenon due to a sharp decrease in the level of protein C. And, despite the anticoagulant function of warfarin, in this situation it provokes a procoagulant status and promotes thrombus formation in small vessels of the skin.
There are two types of protein C deficiency: type I (true, quantitative) occurs most often and is characterized by a decrease in the level of immunological and functional activity of protein C; type II (dysfunctional) - normal immunological and reduced functional activity of protein C.
Heterozygous protein C deficiency occurs in 3.7% of individuals with deep vein thrombosis of the lower extremities and 0.2-0.4% of the general population. Protein C deficiency increases the risk of blood clots by 5-8 times.
Protein C is a vitamin K-dependent glycoprotein. Protein C deficiency is associated with an increased risk of skin necrosis in patients taking warfarin. Protein C has a short half-life of 6 hours compared to other vitamin K-dependent factors. The risk of warfarin skin necrosis does not depend on the nature of the underlying disease and the dose of the indirect coagulant. This complication is most often caused by a deficiency of protein C. Since the T1/2 of protein C is significantly shorter compared to the T1/2 of coagulation factors, and warfarin suppresses the synthesis of all vitamin K-dependent factors, warfarin primarily causes a sharp decrease in the concentration of protein C. This leads to a temporary increase in blood clotting and thrombosis of skin vessels, followed by skin infarction.
As described above, protein C is activated by thrombin bound to thrombomodulin on the surface of endothelial cells. Endothelial protein C/activated protein C receptor (EPCR) is a glycoprotein expressed on the membrane of vascular endothelial cells that binds specifically and with high affinity to protein C and APC. To function, EPCR must be associated with a phospholipid membrane, which stabilizes its three-dimensional structure. Binding of protein C to EPCR enhances its activation by the thrombin-TM complex. EPCR is found mainly on the membrane of large vessels. In addition, it is intensively expressed by syncytiotrophoblast, which prevents the development of thrombosis and plays a certain role in maintaining pregnancy. A soluble form of EPCR (sEPCR) is present in the plasma of some people, which differs from regular EPCR in that it lacks a transmembrane domain and a cytoplasmic tail. sEPCR binds protein C and APC with the same affinity as EPCR, however, this binding to APC inhibits the anticoagulant activity of protein C by blocking the binding of APC to the phospholipid surface, making APC unable to inactivate factor Va. Unlike the membrane-associated form of EPCR, protein C bound sEPCR does not result in increased activation of protein C by the thrombin-TM complex. In contrast, sEPCR-dependent activation of protein C is thrombogenic. Impaired EPCR function can be caused by the presence of mutations/polymorphisms that lead to a decrease in the amount of membrane EPCR (this kind of point mutations are very rare) and mutations/polymorphisms in the EPCR gene that lead to an increased level of the soluble form of EPCR (sEPCR) in the blood. There are about 13 known polymorphisms in the EPCR gene. Polymorphism in the 6936 A/G gene in the EPCR gene is associated with an increased risk of thrombosis, myocardial infarction, and miscarriage. It has also been noted that gene polymorphisms may play a role in the development of malaria infection and are associated with a greater risk of cancer.
Activated protein C resistance (APC-R) refers to the inability of protein C to cleave and inactivate factors Va and/or VIIIa. A variety of triggers can cause protein C resistance, which can be hereditary or acquired. The most common example of a genetically determined APC-R is the factor V Leiden mutation.
For the first time, resistance to activated protein C as a cause of hereditary thrombophilia was described in three different families by the Swedish scientist Dahlbaecketal. in 1993. The consequence of this mutation is disturbances in the functioning of the protein C system, which represents the most important natural anticoagulant pathway. As mentioned above, under normal conditions, APC inhibits coagulation by cleaving a limited number of peptide bonds in both intact and activated factor V (FV/FVa), as well as in factor VIII (FVIII/FVIIIa). APC-dependent cleavage of FVa is stimulated by protein S and proteolytically modified FV under the influence of APC. Thus, normally, factor V potentially mediates two opposing functions: a) procoagulant - after cleavage with thrombin or factor Xa (FXa) and b) anticoagulant - after cleavage with activated protein C (APC). The prothrombotic effect of APC-R in FV Leiden mutation has at least 2 explanations:
- Impaired degradation of FVa by APC, while the procoagulant effect of mutated FVa is maintained.
- Impairment in FVIIIa degradation because normal FV cleavage at Arg506 is required to mediate the synergistic APC cofactor activity of FV along with protein S in degrading factor VIIIa.
Along with the effects of factor VLeiden described above, the effects of this mutation on fibrinolysis are also very significant. The profibrinolytic properties of APC are now well known. The impaired profibrinolytic response to APC in FVLeiden patients is TAFI-dependent. This phenomenon represents one of the important mechanisms of the prothrombotic tendency in patients with the FVLeiden mutation.
Soon after its description, APC resistance began to be detected quite frequently (20-60%) among patients with thrombosis in the Western world. On the contrary, it was not heard of in Asia. The reason turned out to be that the FV:Q506 allele, which causes APC resistance, was found only in European pedigrees (white race), and is absent in the local populations of Asia, Africa, America and Australia. It is believed that a single mutation of the gene encoding factor V occurred about 30,000 years ago, i.e. following the migration of people out of Africa 100,000 years ago and the segregation of Asians from Europeans. This explains the frequency of the mutation in Europe, and its absence in Japan and China, as well as among the local populations of Asia, Africa and America.
The risk of thrombosis with APC resistance is extremely high. Among patients with this complication, the Leiden mutation accounts for 25-40%. With this mutation, the risk of thrombosis is almost 8 times higher than in the absence of the mutation, and with homozygous carriage it is almost 90 times higher.
According to A. Gerhardtetal. (2000), the Leiden mutation was observed in 44% of 119 women with thromboembolic complications during pregnancy.
According to J. Meinardietal. (1999), among 228 mutation carriers, the risk of miscarriage is 2 times higher than in the group of women with miscarriage, but not mutation carriers; 80% of pregnancy losses in mutation carriers occurred in the first trimester and up to 16 weeks.
In a recent study by Bare S.N. et al. (2000) reported that the risk of miscarriage and infertility is 2.5 times higher for carriers of the VLeiden mutation.
Antiphospholipid antibodies (APA) have the ability to inhibit the protein C system in several ways (Fig.):
1) inhibit the formation of thrombin, an activator of protein C (thrombin paradox);
2) inhibit the activation of protein C through its effect on thrombomodulin (antibodies to thrombomodulin);
3) inhibit APC activity (acquired APC resistance), which can be achieved: a) through inhibition of the assembly of proteins of the protein C complex on the anionic surfaces of phospholipid matrices; b) through direct inhibition of APC activity; c) through inhibition of cofactors Va and VIIIa;
4) antibodies affect the levels of protein C and/or protein S (acquired deficiency).
The so-called thrombin paradox is due to the fact that thrombin has both anti- and prothrombotic properties in the hemostatic system. At low concentrations of thrombin, activation of the natural anticoagulant, protein C, occurs predominantly. At this moment, thrombin is an antithrombotic agent. When more thrombin is formed, fibrinogen is converted to fibrin, and FVa and FVIIIa are activated: thrombin exhibits prothrombotic properties. AFAs inhibit the low levels of thrombin formation that are normally observed and reduce circulating levels of activated protein C (APC). After damage to the vascular wall, the level of circulating APC becomes insufficient to prevent uncontrolled thrombus formation, and the hemostatic balance shifts to the prothrombotic side.
Figure 2.
Effect of antiphospholipid antibodies on the protein C system. Antibodies to prothrombin and b2-glycoprotein I disrupt the formation of the prothrombinase complex. This mechanism underlies the phenomenon of lupus anticoagulant. Antiphospholipid antibodies cause the formation of resistance to activated protein C through several mechanisms: disruption of the formation of thrombin - the activator of protein C (thrombin paradox), inactivation of proteins C and S, disruption of thrombomodulin function (antibodies to thrombomodulin), disruption of APC assembly on the anionic phospholipid surface.
Conditions associated with low levels of protein C (acquired deficiency) include:
Warfarin therapy;
Liver diseases (liver cirrhosis);
Vitamin K deficiency;
Fresh thrombosis;
Long-term antibiotic therapy with insufficient food intake;
Metastatic tumors;
DIC syndrome;
Severe bacterial infection at a young age.
In adults, acquired protein C deficiency does not always lead to thrombosis because in these conditions the levels of other clotting factors are also often reduced. In children, acquired protein C deficiency is most often caused by a bacterial infection, especially meningeal infection, and in such conditions the risk of thrombosis is quite high.
Protein C levels are always low in patients taking warfarin. There is no need to determine protein C levels in such patients. If monitoring is necessary, warfarin should be discontinued 14 days before the test. If the risk of thrombosis remains, low molecular weight heparin should be prescribed during warfarin withdrawal. Since protein C is produced in the liver, patients with liver disease also have low levels of protein C.
The concentration of protein C in the plasma of healthy newborns is about 40 IU/dL. In the blood of healthy adults, the normal level of protein C is 65-135 IU/dL.
It seems to us that assessment of the protein C system has important diagnostic and prognostic value for many pathological conditions in obstetric and gynecological practice. In particular, during the circulation of antiphospholipid antibodies and under APS conditions, the formation of antibodies to all components of the protein C system assembly (thrombomodulin, protein S, protein C) can occur; the protein C system is almost always damaged in women with a homozygous or heterozygous form of the factor V Leiden mutation, with hereditary and acquired forms of protein C deficiency. We are talking about such diseases and complications in obstetrics as preeclampsia/eclampsia, PONRP, recurrent miscarriage, premature birth, multiple pregnancy, fetal loss syndrome, thrombosis, thromboembolism, FGR, liver failure, ovarian hyperstimulation syndrome, IVF failure, septic conditions, septic shock, etc.
Unfortunately, to date, in clinical and obstetric practice, the protein C system is not always assessed using screening methods, which results in insufficient information about functional state hemostasis.
Determination of protein C can be performed by various methods:
1) ELISA determination of protein C level (no assessment of functional activity).
2) Determination of the level of antibodies to protein C.
3) Determination of the functional activity of the protein using amidolytic or coagulometric methods (global test). Both functional tests are based on the use of the protein C activator from the venom of the copperhead Agkistrodon contorix. Under the influence of the activator, protein C is activated and, in the presence of its cofactor S, causes proteolysis of factors Va and VIIIa. Therefore, after adding the activator to normal plasma, the clotting time is prolonged. With insufficient protein C, protein S, or APC-R, the elongation is less pronounced. The results are affected by conditions accompanied by vitamin K deficiency (taking indirect anticoagulants, obstructive jaundice and other liver diseases). With a lack of vitamin K, non-carboxylated protein C molecules lose their anticoagulant activity, determined by the coagulometric method, but retain amidolytic and antigenic activity.
In clinical practice, in cases of protein C deficiency, the possibility of replacement therapy with protein C drugs (seprotin, drotrecogin alfa), which have both anticoagulant and profibrinolytic effects (impact on the hemostatic system) and anti-inflammatory and anti-apoptotic effects (cytoprotective effect), has become possible. However, little experience indicates the extremely important need to monitor hemostasis during therapy.
LITERATURE
1. Weiler H. Multiple receptor-mediated functions of activated protein C. // Hamostaseologie. - 2011. - Vol. 31, No. 3 - P. 185-195.
2. Patracchini P., Aiello V., Palazzi P., Calzolari E., Bernardi F. Sublocalization of the human protein C gene on chromosome 2q13-14 // Hum.Genet. - 1989. - Vol. 81. - P. 191-192.
3. Gusina A.A., Gusina N.B. Genetic defects of pro- and anticoagulant proteins as risk factors for venous thrombosis // Medical news. - 2006. - No. 9. - P. 10-14.
4. Gorbacheva L.R. Neuroprotective effect of clinical hemostasis proteinases: abstract. dis. ...Dr.Biol. Sci. - Moscow, 2008. - 49 p.
5. Spek C.A., Reitsma P.H. Genetic risk factors for venous thrombosis // Molecular Genetics and Metabolism. - 2000. - Vol. 71, no. 1-2. - P. 51-61.
6. Larsen T.B., Lassen J.F., Brandslund I., Byriel L., Petersen G.B., Nørgaard-Pedersen B. The Arg506Gln mutation (FV Leiden) among a cohort of 4188 unselected Danish newborns // Thrombosis Research. - 1998. - Vol. 89, No. 5. - P. 211-215.
7. Voetsch B., Loscalzo J. Genetic determinants of arterial thrombosis // Arteriosclerosis, Thrombosis, and Vascular Biology - 2004. - Vol. 24, No. 2. - P. 216-229.
8. Mosnier L.O., Zlokovic B.V., Griffin J.H. The cytoprotective protein C pathway // Blood. - 2007 Apr 15. - Vol. 109, No. 8. - R. 3161-72.
9. Griffin J.H., Mosnier L.O., Zlokovic B.V. Protein C anticoagulant and cytoprotective pathways // International Journal of Hematology. - 2012 Apr. - Vol. 95, No. 4. - R. 333-45. doi:10.1007/s12185-012-1059-0. Epub 2012 Apr 5.
10. Chen X. D., Tian L., Li M., Jin W., Zhang H.-K., Zheng C.-F. Relationship between endothelial cell protein C receptor gene 6936A/G polymorphisms and deep venous thrombosis // Chinese Medical Journal. - 2011 Jan. - Vol. 124, No. 1. - R. 72-75.
11. Saposnik B., Reny J.-L., Gaussem P., Emmerich J., Aiach M., Gandrille S. A haplotype of the EPCR gene is associated with increased plasma levels of sEPCR and is a candidate risk factor for thrombosis // Blood February 15. - 2004. - Vol. 103, No. 4. - R. 1311-1318.
12. Makatsaria A.D., Bitsadze V.O. Thrombophilias and antithrombotic therapy in obstetric practice. - M.: Triada-X, 2003.
13. Dahlbäck B., Carlsson M., Svensson P.J. Familial thrombophilia due to a previously unrecognized mechanism characterized by poor anticoagulant response to activated protein C: prediction of a cofactor to activated protein C // Proc Natl Acad Sci U S A. - 1993 February 1. - Vol. 90, No. 3. - R. 1004-1008.
