PT and INR

The prothrombin time (PT) test measures how long it takes for a clot to form in a sample of blood. In the body, the clotting process involves a series of sequential chemical reactions called the coagulation cascade, in which coagulation or “clotting” factors are activated one after another and result in the formation of a clot. Prothrombin is one of the coagulation factors produced by the liver. One of the final steps of the cascade is the conversion of prothrombin (factor II) to thrombin. The PT test evaluates the integrated function of the coagulation factors that comprise the extrinsic and common pathways of the coagulation cascade, including factors I (fibrinogen), II (Prothrombin), V, VII and X. It evaluates the body’s ability to produce a clot in a reasonable amount of time and, if any of these factors are deficient, the PT will be prolonged.

The PT test is usually measured in seconds and is compared to values in healthy individuals. Because the reagents used to perform the PT test vary from one laboratory to another and even within the same laboratory over time, the normal values also will fluctuate. To standardize results across the U.S. and the world, a World Health Organization (WHO) committee developed and recommended the use of the Internationalized Normalized Ratio (INR) with the PT test for patients who are receiving the blood-thinning medication warfarin (Coumadin). The INR is a calculation that adjusts for changes in the PT reagents and allows for results from different laboratories to be compared. Most laboratories are now reporting both PT and INR values whenever a PT test is performed. However, the INR should not be used to evaluate PT for patients who are not on blood-thinning medications.

A blood sample is obtained by inserting a needle into a vein in the arm or, sometimes, from a fingerstick.

No test preparation is needed. If the patient is receiving anticoagulant therapy, the specimen should be collected before he takes his daily dose.