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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. It lets your doctor understand results in the same way even when they come from different labs and different test methods. In some labs, only the INR is reported and the PT is not reported.
Other blood clotting tests, such as partial thromboplastin time (PTT) and activated clotting time (aPTT), might be used if you take another type of blood-thinning medicine called heparin. These tests measure other clotting factors, or they may be used to see if you are getting the right dose of heparin. 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.
Blood 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.
Health Canada has approved a home test for prothrombin time (PT). If you need a PT test frequently and for a long time, you may want to ask your doctor if this home test is an option for you.
Prothrombin time (PT) is measured to:
Many medicines can change the results of this test. Be sure to tell your doctor about all the non-prescription and prescription medicines you take, as well as any natural health products you use.
The health professional drawing blood will:
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 usually 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 pinch.
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 warfarin (Coumadin) will be the same. In some labs, only the INR is reported and the PT is not reported.
Prothrombin time (PT):
International normalized ratio (INR):
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:
CitationsFischbach F, Dunning MB III (2015). A Manual of Laboratory and Diagnostic Tests, 9th ed. Philadelphia: Wolters Kluwer Health.Other Works ConsultedChernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.Fischbach F, Dunning MB III (2015). A Manual of Laboratory and Diagnostic Tests, 9th ed. Philadelphia: Wolters Kluwer Health.Pagana KD, Pagana TJ (2014). Mosby's Manual of Diagnostic and Laboratory Tests, 5th ed. St. Louis: Mosby.
Current as of: August 31, 2020
Author: Healthwise StaffMedical Review: E. Gregory Thompson MD - Internal MedicineDonald Sproule MDCM, CCFP - Family MedicineKathleen Romito MD - Family MedicineMartin J. Gabica MD - Family MedicineAdam Husney MD - Family MedicineJoseph O'Donnell MD - Hematology, Oncology
Current as of: August 31, 2020
Author: Healthwise Staff
Medical Review:E. Gregory Thompson MD - Internal Medicine & Donald Sproule MDCM, CCFP - Family Medicine & Kathleen Romito MD - Family Medicine & Martin J. Gabica MD - Family Medicine & Adam Husney MD - Family Medicine & Joseph O'Donnell MD - Hematology, Oncology
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