The prothrombin index, the norm of which is 95-105%, is one of the indicators characterizing the blood coagulation system. Blood coagulation is a very complex process, carried out with the participation of a whole cascade of specific proteins called coagulation factors. Activation of this process is possible in two ways - internal and external.
It is the prothrombin index that characterizes the activation of coagulation along the external path. It consists in the following: in case of tissue integrity violation (wound), the so-called tissue thromboplastin, a special protein contained in all tissues of the body, is secreted. It starts a cascade reaction, which other coagulation factors enter (V, VII, as well as X, II, I), sequentially activating each other. Eventually a blood clot forms and the bleeding stops.
What does the prothrombin index say?
Coagulation factors form in the liver. Of these, II, V, X are vitamin K dependent. With a lack of this vitamin, their synthesis is disturbed. Accordingly, there are violations of hemostasis. Knowing the prothrombin index, the norm of which is known, the doctor can:
- Estimate roughly the activity of II, V, also VII and X coagulation factors
- The value of this indicator to judge the function of the liver
- On the saturation of the body with vitamin K
- About the general condition of the blood coagulation system
- By this indicator, you can monitor the effectiveness of anticoagulant therapy.
How is the prothrombin index determined?
The prothrombin index in capillary or venous blood is determined. In the second case, fasting blood is taken into a test tube with an anticoagulant in a ratio of 9: 1. Sodium citrate is used as an anticoagulant. In the laboratory, the blood is centrifuged. The resulting plasma is used to determine prothrombin time (PV). The norm of PV lies in the range from 12 to 20 seconds, depending on the activity of thromboplastin, which is used during the reaction in this laboratory.
Knowing the patient's PV, it is possible to calculate the PI, that is, the prothrombin index, the norm of which in different laboratories may also be slightly different. It is equal to the ratio of the PV of a healthy person to the patient's PV multiplied by 100:
PI = PV health / Pv sick. X 100 (norm 95-105%)
Clinical Importance of PI
With an increase in the coagulation of PV, or prothrombin, whose norm is from 12 to 20 seconds, it shortens, and PI increases. This can be during pregnancy, with massive trauma, when a lot of tissue thromboplastin enters the bloodstream.
On the contrary, PV increases and PI decreases with a decrease in coagulation, for example, in case of congenital insufficiency of coagulation factors, with severe liver diseases, with a deficiency of vitamin K (it can occur with cholestasis, dysbiosis, and impaired bowel function). PI decreases with low fibrinogen or its functional inferiority, with DIC, in the presence of coagulation-inhibiting substances.
What is INR
Treatment with indirect anticoagulants naturally reduces the prothrombin index. Its norm in each laboratory can be its own. Therefore, in order to be able to compare the test results obtained in different laboratories, a standardized test such as INR was introduced. This is a calculated indicator. It is determined based on the ratio of the prothrombin time of the patient, the normal value of PV and the degree of sensitivity index of thromboplastin (MIC).
INR = (PV bth / PV norm) in the degree of MIC
In a healthy person, INR is in the range of 0, 8-1.2.
This indicator is used to monitor the effectiveness of anticoagulants. When treated with anticoagulants, when coagulation decreases (for example, after heart surgery), the INR should be increased to 2-3. With an increase in coagulability, INR decreases. The ratio between PI and INR is such that when the PI decreases, the INR indicator increases.