PT INR – Range, Normal, High, Test Results Interpretation

INR is an abbreviated form for International Normalized Ratio. It is a ratio used to find out the blood clotting ability of an individual.

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INR and the blood clotting pathway: An explanation

There are 12 to 14 factors that aid the process of blood clotting such as tissue glycoproteins and proteins, enzymes, receptors, etc. The occurrence of an internal condition or an external injury to a tissue causes such factors to precipitate, thereby leading a blood clot to form at the given site and thus avoid blood loss. It may be noted that some of these factors are significantly needed in case of internal or external injuries, while other factors are common in the process of a blood clot formation. When the actions of these factors become abnormal, then the risk to increased blood clot formation or excessive bleeding also increases. Hence, it is important to determine the time needed by both the intrinsic and extrinsic pathways to clot the blood.

The test which helps in the measurement of the extrinsic pathway is called the “PT blood test” or “INR PT” or “prothrombin time.” Likewise, the test used to determine the working of the common pathway  and intrinsic factors is known as the “aPTT test”, which is an abbreviated form of “activated partial thromboplastin time.” Doctors prefer to perform both the tests to as to understand the bleeding pattern of an individual.

INR refers to a ratio used for measuring the deviation of an individual’s extrinsic bleeding pattern from that of the common population. It is computed by taking a ratio of PT or an individual’s plasma sample to that of the ‘control sample of plasma’, to the power of International Sensitivity Index or ISI. As per the tissue factor employed in the test, the value of ISI is set. INR can thus be shown as an equation.

Importance of PT INR testing

A doctor may recommend a PT INR test to check for the presence of the following conditions:

  • Deficiency of vitamin K
  • Disseminated Intravascular Coagulation or DIC
  • Warfarin therapy, is considered as one of the most essential reasons for conducting a PT INR test. A PT INR normal values will help in determining the quantity of anticoagulation required by people affected by stroke, thrombosis, atrial fibrillation, pulmonary embolism, and with artificial heart valves
  • Suspected deficiency of factors, like factor X or V
  • Liver failure or end stage liver disorder
  • To check for occurrence of various abnormalities in different platelet and bleeding disorders, such as von Willebrand disease, thrombocytopenia uremia, hemophilia, etc.

The PT INR test

The test is carried out in two steps.

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  • The first step involves collection of the blood sample in an anticoagulant citrate bulb. It may be noted that the citrate found in the bulb has to have a citrate: blood ratio of 1:9. The citrate attaches to calcium and reduces the clotting. Then the blood sample is centrifuged and separation of plasma from the cells is done.
  • The second step involves dropping of the collected plasma onto the analyzer strip and its insertion into the INR automated test machine. The temperature needs to be somewhat near 37 degrees. Increased calcium is then used to wash the blood sample so as to undo the anticoagulation effect of citrate. Clotting is then initiated by adding a standard tissue factor to the sample. The time taken for clotting of blood is then calculated either mechanically by the lab technicians or optically by the machine. This time is then used with the above discussed formula to compute the INR.

Normal values

  • The normal PT ranges from ten to thirteen seconds. As INR is a ratio, the general population will have a value of about 1. After taking into consideration the differences across the general population, the INR ranges between 0.8 and 1.2 among them. This indicates that in most instances, the blood will clot in thirteen seconds on conducting the test.

High PT INR values

A high INR value generally indicates an increased risk of bleeding. Such bleeding is persistent even with small injuries to the blood vessels or the micro vessels. An INR value that is greater than five increases the vulnerability to excessive internal bleeding. Such bleeding of the internal organs can affect their functioning and result in hypotension, ultimately causing death, if not treated immediately.

Individuals with below mentioned abnormalities may have high INR value:

  • Flawed test procedure, particularly collection of more or less blood volume, or using increased citrate amounts
  • Deficiency of Vitamin K
  • Warfarin therapy
  • Mal-absorption syndromes
  • Liver failure or disease

It is important to note that, monitoring of PT INR normal values is very essential after starting out a patient on anticoagulation therapy. Doctors recommend the testing of PT INR normal values on a daily or biweekly basis, during the first month. After the stabilization of the PT INR values, such monitoring can be altered to a weekly checkup and later monthly or twice a month checkup .

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