What is being tested?
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. One of the final steps is the conversion of prothrombin to thrombin. Prothrombin is one of several clotting factors that are produced by the liver. The PT test evaluates the integrated function of these factors and the body’s ability to produce a clot in a reasonable amount of time.

The PT test is usually measured in seconds and is compared to values in healthy individuals. Because the reagents (chemicals) 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 WHO (World Health Organization) 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.

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