The prothrombin time (PT)—along with its derived measures of prothrombin ratio (PR) and international normalized ratio (INR)—are assays evaluating the extrinsic pathway of coagulation. This test is also called “ProTime INR” and “PT/INR”. They are used to determine the clotting tendency of blood, in the measure of warfarin dosage, liver damage, and vitamin K status. PT measures factors I (fibrinogen), II (prothrombin), V, VII, and X. It is used in conjunction with the activated partial thromboplastin time (aPTT) which measures the intrinsic pathway and common pathway.
The result (in seconds) for a prothrombin time performed on a normal individual will vary according to the type of analytical system employed. This is due to the variations between different types and batches of manufacturer’s tissue factor used in the reagent to perform the test. The INR was devised to standardize the results. Each manufacturer assigns an ISI value (International Sensitivity Index) for any tissue factor they manufacture. The ISI value indicates how a particular batch of tissue factor compares to an international reference tissue factor. The ISI is usually between 0.94 and 1.4 for more sensitive and 2.0-3.0 for less sensitive thromboplastins.
The INR is the ratio of a patient’s prothrombin time to a normal (control) sample, raised to the power of the ISI value for the analytical system being used.
Content under Creative Commons Attribution-ShareAlike 3.0 Unported License