Prothrombin time

Reference values ​​of prothrombin time: adults - 11-15 seconds, newborns - 13-18 seconds.

Prothrombin time characterizes the I and II phases of plasma hemostasis and reflects the activity of the prothrombin complex( factors VII, V, X and prothrombin itself - Factor II).

The increase in prothrombin time indicates a tendency to hypocoagulation and may depend on various causes.

■ Insufficiency of one or more factors of the prothrombin complex observed in such hereditary coagulopathy as hypoproconvertinemia( factor VII deficiency) and hypoprothrombinemia( Factor II deficiency).

■ Occasionally, in amyloidosis, an increase in prothrombin time is associated with a deficiency of factor X, which is absorbed by amyloid, and in nephrotic syndrome - deficiency of factors VII and V, which are excreted in the urine.

■ Synthesis of factors of the prothrombin complex occurs in liver cells, with the latter the number of them decreases, so prothrombin time to a certain extent can serve as an indicator of the functional state of the liver. An increase in pro-thrombin time is noted in acute and chronic hepatitis, cirrhosis of the liver, subacute liver dystrophy and other lesions of the liver parenchyma, which is considered a poor prognostic sign. In this case, the cause of the increase in prothrombin time may be a malabsorption of vitamin K, which develops as a result of a decrease in the intake of bile in the intestine, which is necessary for the synthesis of the prothrombin complex factors. The same is the cause of the increase in prothrombin time and with mechanical jaundice.

■ Enteropathy and intestinal dysbiosis, leading to vitamin K deficiency, may also be accompanied by an increase in prothrombin time.

■ In the treatment of vitamin K antagonists( indirect anticoagulants), the final stage of the synthesis of factors of the prothrombin complex is disrupted, and the prothrombin time is prolonged.

■ Consumption of factors of the prothrombin complex with acute DIC syndrome leads to a rather early increase in prothrombin time( 2 times or more).

■ In chronic pancreatitis, pancreatic cancer and gallbladder, the increase in prothrombin time can be the result of liver damage and / or the development of DIC syndrome.

■ Afibrinogenemia, hypofibrinogenemia( decrease in fibrinogen in the blood up to 1 g / l and below), as well as excessive heparin content in the blood lead to an increase in prothrombin time.

■ Prothrombin time is elongated in acute and chronic leukemia, due to the development of DIC syndrome.

■ An increase in the concentration of antithrombin or antithromboplastin in the blood also leads to lengthening of prothrombin time;

■ A whole group of drugs can prolong prothrombin time: anabolic steroids, antibiotics, acetylsalicylic acid( in high doses), laxatives, methotrexate, nicotinic acid, chi-nidin, thiazide diuretics, tolbutamide.

The shortening of prothrombin time indicates a tendency to hypercoagulable and can be noted in the initial stages of deep vein thrombosis of the lower extremities, with polycythemia, in the last months of pregnancy. The shortening of prothrombin time is caused by the following drugs: acetylsalicylic acid( in small doses), mercaptopurine, oral contraceptives.

The determination of prothrombin time is given the leading role in the control of anticoagulant therapy, however, with this control, the prothrombin time depends on the sensitivity of the thromboplastin used for these purposes. Therefore, comparing the results of studies using various thromboplastins is an important task of practical medicine. Different thromboplastins are distinguished by ISI [International Sensitivity Index( International Sensitivity Index)], which is attached in the description of each set. In 1983, WHO, together with the International Society of Thrombosis and Hemostasis, was taken for reference thromboplastin from the human brain and found that the ISI of this thromboplastin is 1( the International Reference Preparation of the World Health Organization).All other commercial thromboplastins are calibrated for it, and each has its own sensitivity( MIC).To compare the results of prothrombin time studies in patients receiving anticoagulant therapy, it is necessary to calculate INR [International Normalized Ratio( INR)).INR( INR) =( Prothrombin ratio) w;Prothrombin ratio( PTR) = prothrombin time hurts

go( s) / prothrombin time control( s).MNO - an attempt to mathematically adjust the difference in the results of the study, related to the different sensitivity of thromboplastins, that is, the reduction of the result to the data obtained with reference thromboplastin( Table).

Table Normalization of prothrombin time with the use of four different thromboplastins according to the calculation of the INR

Table Normalization of prothrombin time using four different thromboplastins according to the calculation of the INR

Laboratories recommend the use of thromboplastins with PMI less than 1.5.In rabbit thromboplastins, the MIC is 2-3.In the United States, all laboratories switched to placental human thromboplastin, whose MIC is 1 [Mammen E. F., 1996].

The main task of monitoring the intake of oral anticoagulants is to prevent bleeding. Until recently, it was recommended to support the prothrombin time of when treated with indirect anticoagulants 2-2.5 times longer than normal( rabbit thromboplastin).However, this time was too long, which often led to bleeding. Currently, WHO has developed recommendations for the control of anticoagulant therapy, expressed in INR( Table).

Table Indicators of INR recommended for therapy with anticoagulants

Table Indicators of INR recommended for anticoagulant therapy

For convenience in calculating INR, we give a scale on which the dependence of INR on MIC and PTR is presented( Table).The vertical scale on the left shows the values ​​of PTR( the ratio of the prothrombin time of the patient to the prothrombin time of the control plasma), and the horizontal scale on the top shows the MIC values ​​(for various thromboplastins).At the intersection of the lines of these two parameters is the INR for this patient.

Table Scale for determination of INR( MP - prothrombin time)

Table Scale for determination of INR( PV - prothrombin time)