What is a PT/INR?
Prothrombin is a substance in the blood produced by the liver. The prothrombin time test (commonly known as the pro-time or PT) measures the time required for a clot to form in a tube of your blood after the addition of chemicals to speed up the clotting reaction artificially. Normally the blood clots in 10-14 seconds in a person who does not take warfarin. It should take your blood 2-3 times longer to clot than the “normal” sample. The International Normalized Ratio (INR) system is the best means of standardizing the measurement of prothrombin time to monitor warfarin therapy. It is a calculation using the prothrombin time to make adjustments for the varying chemical sensitivities used at different laboratories. This test is done by either venipuncture or by an instrument that provides an INR score from a blood sample taken from a finger stick.
