Antithrombin III -protein that inactivates blood clotting factors, including thrombin, and prevents excessive formation of blood clots. Quantitative and qualitative studies of antithrombin III in the blood are performed within the framework of a coagulogram, the interpretation of the indicator is often made taking into account the results of tests for protein C, protein S and lupus anticoagulant. Antithrombin III test is necessary for the diagnosis and control of therapy, including heparin therapy, of diseases accompanied by increased formation of blood clots or bleeding: thrombosis, DIC syndrome, congenital deficiency of antithrombin III, liver diseases, oncological pathologies. Venous blood is used for the study, the colorimetric method is used. In adults, the normal indicators range from 66% to 124%. The readiness of the analysis results is 1 working day.
Antithrombin III is a glycoprotein that is synthesized in hepatocytes and in the vascular endothelium. It is the main endogenous anticoagulant, that is, it helps to maintain blood in a liquid state. Antithrombin III acts as an inhibitor of plasma coagulation factors: it reduces the activity of thrombin, coagulation factors VIIA, IXA, XA, XIA, XIIA. This mechanism helps to prevent excessive formation of blood clots. In addition, antithrombin III is a plasma cofactor of heparin. When it acts independently, thrombin inactivation occurs gradually, incrementally, and when heparin enters the blood, this process occurs quickly. Patients with low levels of antithrombin III have a high risk of developing thrombosis, and heparin has virtually no anticoagulant effect. With hereditary deficiency of this glycoprotein, thrombotic symptoms appear already at the age of 20-25 years.
The material for studying the level of antithrombin III is plasma isolated from venous blood. The test is performed by kinetic colorimetric method using automatic (or semi-automatic) analyzers. The results are widely used in hematology, surgery, obstetrics and gynecology, resuscitation. A blood test for antithrombin III allows you to identify an innate and acquired tendency to thrombosis, evaluate the effectiveness of therapy for diseases of this group, as well as identify the risk of hemorrhagic complications during therapy with direct anticoagulants, after surgery, childbirth, trauma.
The main indication for a blood test for antithrombin III is a hereditary deficiency of this glycoprotein. Its insufficiency is transmitted by autosomal dominant type, manifested by severe recurrent thrombosis of veins and arteries, myocardial infarction, stroke at the age of 30-35 years with heterozygous form, from birth – with homozygous. Acquired deficiency of antithrombin III develops against the background of liver diseases (cirrhosis, cancer), nephrotic syndrome, DIC syndrome, cancer, transfusions of large volumes of blood, surgical interventions.
To determine the causes of increased thrombosis, the antithrombin III test is carried out together with the study of prothrombin, proteins C and S, lupus anticoagulant. The ineffectiveness of heparin therapy may be the basis for the appointment of an analysis, since it is caused by a deficiency of antithrombin III (heparin cofactor). The study is also used to monitor the blood clotting system in patients taking anticoagulants in order to prevent thromboembolic complications with prolonged restriction of movements, operations, injuries, childbirth.
A blood test for antithrombin III is not indicated for patients with symptoms of inflammatory and infectious diseases – acute respiratory infections, influenza, rhinitis, pharyngitis, rheumatoid arthritis, acute pancreatitis and others. In such cases, blood clotting increases, the level of antithrombin III decreases, without displaying the real indicator. Contraindications for blood sampling are mental and motor arousal of the patient, severe anemia, hypotension. The antithrombin III test is highly sensitive, but it does not indicate the cause of the deficiency of this glycoprotein. Therefore, the analysis is always carried out in conjunction with other coagulogram tests, as well as clinical and instrumental studies.
Preparation for analysis and material collection
In the study of antithrombin III, venous blood is the material. Its fence is carried out in the morning, on an empty stomach. The last meal should be carried out no later than 10-12 hours before the procedure. In 30 minutes, you need to give up smoking, physical activity, and avoid emotional stress. It is necessary to warn the doctor who issues the referral for the study and the laboratory assistant about all medications taken. It is also worth remembering that the inflammatory process in the body can distort the results, so if you are unwell, it is recommended to postpone the procedure to a later date, until full recovery.
As a rule, blood for examination is taken from the ulnar vein. It is collected in a test tube with sodium citrate, an anticoagulant that prevents the formation of clots. Within 4-6 hours, the material is delivered to the laboratory, where it is centrifuged to produce plasma. Most often, the study is carried out by the kinetic colorimetric method. Thrombin is injected into the diluted plasma, after incubation, the material is poured onto a chromogenic substrate. The analyzer measures the absorption of monochrome light passing through the cuvette. This indicator is directly proportional to the level of antithrombin III in the sample. The readiness of the analysis results is no more than 1 working day.
In adults, the normal level of antithrombin III in the blood ranges from 66 to 124%. During the newborn period, it is lower – 58-90%, in the first year of life – 72-134%, from a year to 6 years – 101-131%, from 6 to 11 years – 95-134%, from 11 to 16 years – 66-126%. During pregnancy, starting at 13 weeks, the level of antithrombin gradually decreases, as the body prepares for childbirth and possible blood loss. By the end of the term, the indicators normally do not exceed 116%. A physiological decrease in the amount of antithrombin III in the blood occurs in men with age, in women during menopause, during menstruation, an increase in the level of this protein is possible. It is worth remembering that the range of reference values in laboratories may vary, as it depends on the equipment and reagents used. In order to obtain reliable information, it is necessary to compare the actual results with the norm specified in the form.
Raising the level
When the level of antithrombin III increases, the blood clotting process is activated. The cause may be acute hepatitis, acute cholestasis, pancreatic cancer, severe acute pancreatitis, kidney transplantation. The amount and activity of antithrombin III increases with vitamin K deficiency, which can be caused by blockage of the bile ducts, malabsorption syndrome, cystic fibrosis, resection of the small intestine. A gradual steady increase in indicators occurs with prolonged use of indirect anticoagulants, for example, warfarin or phenylin, as well as anabolic steroids.
Lowering the level
The reason for the decrease in the level of antithrombin III in the blood is a violation of its formation in the liver or increased consumption. Insufficient synthesis of this protein may be caused genetically (congenital deficiency) or caused by severe liver diseases: cirrhosis, liver failure, viral hepatitis, Budd-Chiari syndrome, organ transplantation or partial removal. A decrease in the level of antithrombin III in the blood is determined with DIC syndrome, deep venous thrombosis, thromboembolism, sepsis, nephrotic syndrome, hypoproteinemia, atherosclerosis, as well as with large blood loss as a result of operations, injuries, childbirth, perforation of the walls of the gastrointestinal tract. A temporary decrease in antithrombin III in the blood occurs when heparin is administered, oral contraceptives are taken.
Treatment of abnormalities
Antithrombin III refers to primary anticoagulants, helps to maintain blood in a liquid state. Analysis to determine its level and activity is used in different areas of clinical practice: where an assessment of the anticoagulation system of the blood is necessary. It is performed within the framework of an extended coagulogram. If the indicators deviate from the norm, a hematologist is most often engaged in prescribing treatment, in some cases – a hepatologist (gastroenterologist), a surgeon or an obstetrician-gynecologist. Physiological deviations from the norm associated with age, pregnancy or menstrual cycle do not require special correction. To obtain reliable results of the analysis, it is necessary to exclude taking medications that affect blood clotting.