Prothrombin Time

International Normalized Ratio (INR), Pro Time, PT (Prothrombin Time)

Results

Prothrombin time (PT) is a blood test that measures how long it takes blood to clot.

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 so the results among labs using different test methods can be understood in the same way. Using the INR system, treatment with blood-thinning medicine (anticoagulant therapy) will be the same. In some labs, only the INR is reported and the PT is not reported.

Prothrombin time (PT)

Normal:	10–13 seconds
International normalized ratio (INR):	1.0–1.4

The warfarin (Coumadin) dose is changed so that the prothrombin time is longer than normal (by about 1.5 to 2.5 times the normal value or INR values 2 to 3). Prothrombin times are also kept at longer times for people with artificial heart valves, because these valves have a high chance of causing clots to form.

Abnormal values

• A longer-than-normal PT can mean a lack of or low level of one or more blood clotting factors (factors I, II, V, VII, or X). It can also mean a lack of vitamin K; liver disease, such as cirrhosis; or that a liver injury has occurred. A longer-than-normal PT can also mean that you have disseminated intravascular coagulation (DIC), a life-threatening condition in which your body uses up its clotting factors so quickly that the blood cannot clot and bleeding does not stop.
• A longer-than-normal PT can be caused by treatment with blood-thinning medicines, such as warfarin (Coumadin) or, in rare cases, heparin.

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Author:	Robin Parks, MS	Last Updated: September 15, 2008
Medical Review:	Anne C. Poinier, MD - Internal Medicine Brian Leber, MDCM, FRCPC - Hematology
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