Prothrombin time (PT) is a blood test that measures how
long it takes blood to clot. A prothrombin time test can be used to check for
bleeding problems. PT is also used to check whether medicine to prevent blood
clots is working.
A PT test may also be called an INR test. INR
(international normalized ratio) stands for a way of standardizing the results
of prothrombin time tests, no matter the testing method. So your doctor can
understand results in the same way even when they come from different labs and
different test methods. 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.
clotting factors are needed for blood to clot
(coagulation). Prothrombin, or factor II, is one of the clotting factors made
by the liver. Vitamin K is needed to make prothrombin and other clotting
factors. Prothrombin time is an important test because it checks to see if five
different blood clotting factors (factors I, II, V, VII, and X) are present.
The prothrombin time is made longer by:
An abnormal prothrombin time is often caused by liver
disease or injury or by treatment with blood thinners.
blood clotting test, called partial thromboplastin time (PTT), might be used if you take another type of blood-thinning medicine called heparin. This test measures other
clotting factors. Partial thromboplastin time and prothrombin time are often
done at the same time to check for bleeding problems or the chance for too much
bleeding in surgery.
Prothrombin time (PT) is measured
Many medicines can change the results
of this test. Be sure to tell your doctor about all the nonprescription and
prescription medicines you take, as well as any supplements or herbal remedies
The health professional drawing blood
In some cases the health professional will take a sample of blood from your fingertip instead of your vein. For a finger stick blood test, the health professional will clean your hand, use a lancet to puncture the skin, and place a small tube on the puncture site in order to collect your blood.
The blood sample is taken from a vein in
your arm. An elastic band is wrapped around your upper arm. It may feel tight.
You may feel nothing at all from the needle, or you may feel a quick sting or
There is very little chance of a problem from
having a blood sample taken from a vein.
Prothrombin time (PT) is a blood test
that measures how long it takes blood to clot.
The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your 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.
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.
Reasons you may not be able to
have the test or why the results may not be helpful include:
CitationsPagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.Other Works ConsultedChernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
September 6, 2012
E. Gregory Thompson, MD - Internal Medicine & Joseph O'Donnell, MD - Hematology, Oncology
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