PT
At a Glance
Why Get Tested?
To check how well blood-thinning medications (anti-coagulants) are working to prevent blood clots
When to Get Tested?
If you are taking an anti-coagulant drug or think you may have a bleeding disorder
Sample Required?
A blood sample taken from a vein in the arm
The Test Sample
What is being tested?
The test measures how long it takes for your blood to begin to form clots. Prothrombin is a plasma protein produced by the liver. Clotting is caused by a series of chemical reactions, including the conversion of prothrombin to thrombin. The test used to measure this clotting factor is called Prothrombin time, Pro time or PT.
How is the sample collected for testing?
Blood is collected by needle from a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
The Test
How is it used?
The Prothrombin time (PT) test, standardised as the INR test is most often used to check how well anti-coagulant or "blood-thinning" tablets such as warfarin and phenindione are working. Anti-coagulant tablets help prevent the formation of blood clots (they do not "thin the blood" as is popularly thought). This is particularly important in people with heart conditions such as atrial fibrillation or artificial valves, or people with a history of recurrent blood clots. The drug's effectiveness can be determined by how much it prolongs the PT (measured in seconds), or increases the INR (a standardised ratio of the patient's PT versus a normal sample).
When is it requested?
If you are taking an anti-coagulant drug, your doctor will check your INR regularly to make sure that your prescription is working properly and that the INR is appropriately increased. There is no set frequency for doing the test. Your doctor will order it often enough to make sure that the drug dose is right. Occasionally the PT or INR may be used on a patient who is not taking anti-coagulant drugs—to check for a bleeding disorder, liver disease or vitamin K deficiency, or to ensure clotting ability before surgery.
What does the test result mean?
The test result for PT depends on the method used; results will be measured in seconds.
Most laboratories report PT results that have been adjusted to the International Normalized Ratio (INR). Patients on anti-coagulant drugs usually have a target INR of 2.0 to 3.0 (i.e. a prothrombin time 2 to 3 times as long as in a normal patient, using standardized conditions). For some patients who have a high risk of clot formation, the INR needs to be higher: about 3.0 to 4.0. Your doctor will use the INR to adjust your drug to get the PT into the range that is right for you. An increased Prothrombin time or INR means that your blood is taking longer to form a clot. If you are not taking anti-coagulant drugs and your PT is prolonged, additional testing may be necessary to determine the cause.
Is there anything else I should know?
Some substances you consume—such as alcohol and different drugs - can interfere with the PT test and give a misleading result. Antibiotics, aspirin, and cimetidine can increase PT. Barbiturates, oral contraceptives and hormone-replacement therapy (HRT), and vitamin K - either in a multivitamin or liquid nutrition supplement - can decrease PT. Make sure that your doctor knows all the drugs you are taking so that the PT results are interpreted correctly.
Common Questions
Ask a Laboratory Scientist
Article Sources
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
