Prothrombin time (PT) is a measure of how long it takes blood to clot. At least a dozen blood proteins, or blood clotting factors, are needed to clot blood and stop bleeding (coagulation). Prothrombin, or factor II, is one of several clotting factors produced by the liver. Adequate amounts of vitamin K are needed to produce prothrombin.

Prothrombin time is an important coagulation test because it measures the presence and activity of five different blood clotting factors (factors I, II, V, VII, and X). The prothrombin time will be lengthened by:

• A low amount of blood clotting factors.
• An absence of any of the factors.
• A decrease in the activity of any of the factors.
• The presence of a substance that inhibits any of the factors.

A prothrombin time test can be used to screen for bleeding abnormalities. Partial thromboplastin time (PTT) is another blood clotting test that measures several other clotting factors. Partial thromboplastin time is often measured along with prothrombin time to evaluate bleeding abnormalities. These two tests together screen for problems with the normal blood clotting process and can reveal most blood clotting problems due to abnormal amounts of coagulation factors. 

A prothrombin time test may also be used to monitor treatment with medication that prevents the formation of blood clots.

An abnormal prothrombin time is often caused by liver disease or injury or by treatment with the medication warfarin (Coumadin), which is used to prevent the formation of blood clots.

This test is done on a blood sample taken from a vein.

Prothrombin Time
Results

Normal

Normal values may vary from lab to lab.

A method of standardizing prothrombin time results, called the international normalized ratio (INR) system, has been developed to compare prothrombin time results among labs using different test methods. Using the INR system, treatment to prevent blood clots (anticoagulant therapy) remains consistent even if a person has the test done at different labs.

People taking warfarin (Coumadin) to prevent blood clots from forming usually keep their prothrombin times about 1.5 to 2 times the normal value (or INR values 2 to 3). Also, people with artificial heart valves may keep their INR at higher levels to prevent clots from forming.

Greater than normal values may mean

• A long prothrombin time can indicate a lack of one or more blood clotting factors (factors I, II, V, VII, or X), a lack of clotting factor activity, a vitamin K deficiency, liver disease, or injury.
• A long prothrombin time can be caused by treatment with certain medications, such as heparin or warfarin (Coumadin), that are used to prevent the formation of blood clots.
• A long prothrombin time can be caused by cirrhosis.
• A long prothrombin time can also be caused by rare inherited clotting factor deficiencies.