Prothrombin time

Prothrombin is a protein made by the liver. Prothrombin helps blood to make normal clots. The "prothrombin time" (PT) is one way of measuring how long it takes blood to form a clot, and it is measured in seconds (such as 13.2 seconds). A normal PT indicates that a normal amount of blood-clotting protein is available.

When the PT is high, it takes longer for the blood to clot (17 seconds, for example). This usually happens because the liver is not making the right amount of blood clotting proteins, so the clotting process takes longer. A high PT usually means that there is serious liver damage or cirrhosis.

• Some patients take a drug called Coumadin (warfarin), which elevates the PT for the purpose of "thinning" the blood. This is not related to having liver disease because it is the Coumadin causing the PT to be high.
• The test called INR measures the same factors as PT and is used instead of PT by many doctors. See "INR."