Prothrombin Time (PT) and International Normalized Ratio (INR)

Blood coagulation is a complex process involving different types of factors and proteins. It helps in stopping bleeding (haemostasis). Several proteins, factors, and enzymes work inconjunction with sequential steps. It is to ensure the formation of a blood clot as soon as bleeding begins after an injury. In order for clotting to happen, an adequate quantity of each of the protein, enzyme, and factor is essential and each single element of the cascade must work properly. PT (Prothrombin Time) is a test that helps detect the cause of excessive bleeding.

INR: International normalized ratio is obtained from the result of PT. It is helpful in assessing the condition of a person undergoing treatment with warfarin – an anticoagulant or blood-thinning medicine.

Why is the test performed?

A PT along with PTT (partial thromboplastin time) may help a physician determine the reason for blood clots and unexplained bleeding. A PT and INR test helps in monitoring the effects of warfarin-an anticoagulant drug. The tests may also be used as pre-surgical screening tests to look for possible bleeding disorders in individuals who may undergo surgery.

When is the test ordered?

A healthcare provider may order a PT when he or she suspects that the person being examined has the signs and symptoms of abnormal bleeding or clotting even thoughthe patient is not on anticoagulant therapy. Such symptoms may include easy bruising or excessive bleeding, bleeding gums, and nosebleeds. The test may also be ordered if the symptoms are associated with any other underlying acute or chronic health condition, such as disseminated intravascular coagulation (DIC) or liver disease.

How is the sample collected?

A blood sample is drawn from a vein in the arm.

How to prepare for the test?

No special preparation is required for the test.

Interpretation of test results

PT and INR is not a diagnostic test by itself as it only indicates some abnormality. It suggests that further testing may be required to confirm the definitive cause and diagnosis.

Therefore, in addition to the PT and INR or along with this test, the other tests that may be ordered include PTT, fibrinogen testing, platelet count, and coagulation factors tests.

High INR or prolonged PT: This may indicate vitamin K deficiency, warfarin therapy, defective factor VII and liver disease.

PT INR Normal values or PT INR Normal range

In healthy young individuals: 1.1 or below

2.0 to 3.0 (for people who are taking warfarin)

The normal INR values for individuals at a very high risk of developing a blood clot is a little bit on a higher side: 2.5 to 3.5.

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