Also known as: Prothrombin Time Assay, Protime, PT.

The PT test is usually measured in seconds and compared to the values in healthy individuals. Because the reagents used to perform the PT test vary from laboratory to laboratory and even within the same laboratory over time, the normal values also will fluctuate. In order 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 then be compared. Both the PT and INR values are reported on this test.