Prothrombin time

Reference values (norm) prothrombin time: adults - 11-15 seconds, newborns - 13-18 with.

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 definition 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) ^ISI; Prothrombin ratio (PTR) = prothrombin time of patient (s) / prothrombin time of control (c).

MNO is an attempt to mathematically correct 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.

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

Thromboplastin ISI	Prothrombin time, s		Calculated values
	A patient	Control	PTR	INR
1.2	24	Eleven	2.2	2.6
3.2	16	12	1.3	2.6
2.0	21	13	1.6	2.6
1.0	38	14.5	2.6	2.6

Laboratories are recommended to use 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.

The main task of monitoring the intake of oral anticoagulants is to prevent bleeding. Until recently, it was recommended to maintain prothrombin time in the treatment of 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.

Indices of INR recommended for anticoagulant therapy

Clinical status	Recommended INR
Prophylaxis of deep vein thrombosis	2-3
Treatment of deep vein thrombosis and pulmonary thromboembolism	2-3
Recurrent deep vein thrombosis, pulmonary thromboembolism	2-3
Prosthetic heart valves made from own tissue	2-3
Mechanical prosthetic heart valves	2.5-3.5
Recurrent deep vein thrombosis and pulmonary embolism	3-4.5
Vascular diseases, including myocardial infarction	3-4.5
Recurrent systemic embolism	3-4.5

For the convenience of calculating INR, we give a scale on which the dependence of INR on MIC and PTR is presented. 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.

[1], [2], [3], [4], [5], [6]