Inr in blood


For problems related to blood clotting, as well as an increase in the risk of blood clots, oral coagulants are prescribed for treatment. In this case, it is very important to conduct a regular analysis of blood clotting for a possible adjustment of the dosage of drugs. Consider what is the INR in the blood and its rate.

Why measure the rate of blood clotting?

INR in the blood: the norm and transcript analysis

Measurement of blood coagulation rate in certain conditions is possible with the help of such indicator as prothrombin time (PTV). This is one of the most simple laboratory tests, which allows you to accurately assess the effectiveness of coagulation. Measurement of prothrombin time is mainly used to monitor the condition of people while taking oral anticoagulants - vitamin K antagonists, such as "warfarin".

In practice, the prothrombin time is the period during which the plasma without platelets begins to coagulate with the addition of special reagents - thromboplastin and calcium. Prothrombin time is expressed in seconds or percent relative to the calibration curve.

However, partly due to the different measurement methods and due to the different activity of the reagents used, the results vary greatly even in the same patient. To standardize the results, a reagent calibration system was developed depending on the different quality of commercial thromboplastin.

This was done to calculate a coefficient called the International Normalized Ratio (INR), which takes into account the patient's clotting time and reference reagents. To date, the results of measurement of prothrombin time in patients while receiving anticoagulants are expressed in the INR.

This analysis is primarily used to assess the degree of blood coagulation, for example, in case of its violation, as well as for the therapeutic monitoring of patients receiving vitamin K antagonists. The reduction in prothrombin time in the blood indicates that it is less prone to blood clots. Conversely, an increase in prothrombin time increases the risk of blood clots and, therefore, the development of thrombosis.

How do the analysis?

measurement targets of the INR

For analysis, venous blood taken from a vein in the elbow, preferably on an empty stomach, is used. The cases of taking anticoagulants shortly before taking a blood sample should be notified to the doctor. Samples are placed in a sodium citrate tube to prevent premature blood clotting. After sample treatment with the reagent, prothrombin time is measured using automated equipment.

So, find out the purpose of measuring the INR:

  • The boundary values โ€‹โ€‹of this coefficient are the main indicators of the rate of blood clotting in people taking vitamin K antagonists.
  • The value of the INR coefficient is decisive in the selection of the dose of anticoagulant.
  • Depending on the individual characteristics of the patient, the personal limits of the INR of the therapeutic range are determined by the doctor. In the future, the INR is maintained in the specified range, which provides an effective treatment.
  • For timely adjustment of the dose of drugs should be regularly monitored INR.

INR in blood: normal

The prothrombin time is the period between the addition of thromboplastin to the blood sample and the formation of a clot - coagulation. As a rule, in the absence of pathologies, this time interval is about 10-11 seconds. The prothrombin time, expressed as a percentage, usually ranges from 70 to 100%, which corresponds to the INR = 1.

In people who receive oral anticoagulants, the target value of INR, which reflects good therapeutic efficacy, is usually 2.5. But the interval may vary depending on the specific situation. The effective therapeutic range that must be maintained is determined in terms of thromboembolic risk. This zone corresponds to the INR when receiving "Warfarin" and is in the range from 2 to 4.5 or in units of prothrombin time from approximately 20 to 40%.

An abnormally large value of prothrombin time in the absence of treatment with anticoagulants may indicate the existence of a hereditary deficiency of one or several blood clotting factors or a very low level of fibrinogen โ€” less than 0.8 g / l.

The result of a blood test for INR and PTI is normal in humans

The transcript of the blood test and the INR standards are presented in the table.

Specific situation



In a healthy person



Prevention of venous thrombosis



Phlebitis or developing thromboembolism



Prevention of recurrent thrombosis



Prevention of arterial thrombosis



Surgical prevention



Heart Prosthesis Patient



In the countries of the former Soviet Union and today, instead of the INR coefficient, the prothrombin index (PTI) can be used to express the degree of blood clotting. The index value is the ratio of the PTV of the reference sample and the blood collected for analysis.

PTI is not universal. There is more than one method for determining the index; moreover, it significantly depends on the activity of the reagent added to the plasma, which is different for each manufacturer. Therefore, the results of the analysis are significantly different in different laboratories.

The result of a blood test for INR and PTI is normal in a person without pathological deviations, expressed as a percentage and ranges from 70 to 100. A low PTI increases the risk of bleeding, a high one contributes to thrombosis.

The use of coagulants reduces blood clotting, so their uncontrolled use can lead to significant blood loss. It is very important to observe the dosage during treatment and regularly monitor blood coagulation.