A prothrombin time (PT) test uses blood samples to measure how quickly your blood forms a clot. Healthcare providers often do this test to monitor your prothrombin levels if you’re taking the blood thinner warfarin. This test is also used to investigate potential blood disorders. A high PT level means your body takes more time than normal to form blood clots.
A prothrombin time (PT) test uses blood samples to measure how quickly your blood forms a clot. If you’re injured and bleeding, your body launches a step-by-step process that creates clots that stop the bleeding. That process involves proteins, called clotting factors or coagulation factors. Prothrombin is one of several clotting factors that combine forces to create blood clots. But the clotting process doesn’t work if there’s not enough of each clotting factor and not all factors function as they should. A high PT level means it takes more time than usual for your body to form clots.
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Healthcare providers often do this test to monitor your prothrombin levels if you’re taking the blood thinner warfarin (Coumadin®). Warfarin helps prevent blood clots, which can cause serious conditions such as deep venous thrombosis or pulmonary embolism.
There are several reasons why your healthcare provider may do a prothrombin time test:
If you’re taking a prothrombin time test, it’s because your healthcare provider is gathering information so they can assess your situation. A PT test is an early step in that process. It’s understandable if you have questions. Ask your healthcare provider to explain the test process and objective. They’ll be glad to explain what this test may show and how that information may be helpful.
Healthcare providers use PT tests to check prothrombin levels, one of the coagulation factors that help form blood clots. A partial thromboplastin time test checks your plasma for coagulation factor abnormalities.
Some medications, supplements, food and alcohol may affect test results. Tell your healthcare provider if you’re taking certain medications or supplements. Ask your healthcare provider what you should avoid before your test. Examples of medications that may affect PT test results include:
If you’re taking warfarin, vitamin C and K supplements may affect your test results.
Foods that contain large amounts of vitamin K can affect results, including:
A prothrombin time test is a simple procedure. To do this test, your healthcare provider will swab your skin with alcohol. Then, they’ll place an elastic band around your upper arm and ask you to make a fist. Making a fist helps your blood flow more easily. You may feel a sting or prick when your healthcare provider inserts the needle. After your healthcare provider has finished drawing blood, they’ll put a small bandage on spot where they inserted the needle.
Typically, people receive their test results within a few hours to one day. Your healthcare provider will explain what the results mean and whether you’ll need additional tests.
Most laboratories report your PT test results as the international normalized ratio, or INR. Your INR result is a calculated measurement that adjusts for differences in laboratory testing processes.
Your test result will show different ranges for PT and INRs The normal ranges for clotting are:
If your INR is above 1.1, your blood is clotting more slowly than normal. Depending on your situation, your healthcare provider may order additional tests so they can diagnose and treat the problem.
Several things can affect prothrombin time test results for people taking warfarin. You may have high PT/IR results if:
A note from Cleveland Clinic
If you’re injured and bleeding, your body races to form blood clots to stop the bleeding so you can begin to heal. When that doesn’t happen, healthcare providers use a prothrombin time (PT) test to find out why. You may have regular PT tests because you’re taking the blood thinner warfarin (Coumadin). Some people have PT tests as a first step toward diagnosing conditions, including blood disorders. Your PT test results will show if your prothrombin levels are normal or too high. You may have questions about what’s next. Your healthcare provider will have answers.
Last reviewed by a Cleveland Clinic medical professional on 03/14/2022.
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