Prothrombin -Time
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Prothrombin Time Test (PT/INR)
A prothrombin time (PT/INR) test measures how fast a blood sample forms a clot. Healthcare providers often do this test to monitor how fast your blood clots if you’re taking the blood thinner warfarin. It’s also used to diagnose blood disorders. A high PT/INR means your body takes longer than normal to form blood clots.
Overview
A prothrombin time (PT or PT/INR) test measures how quickly your blood clots. The results may be in seconds or, more commonly, a calculation called the International Normalized Ratio (INR).
When you’re bleeding, proteins called clotting or coagulation factors work together to form clots that stop blood loss. Prothrombin is one of several clotting factors that’s essential to this process. It’s a protein your liver makes.
Providers often perform PT tests alongside another blood clotting test called a partial thromboplastin (PTT) test. A PTT test measures how other clotting factors in your blood are working.
When is a PT test performed?
Healthcare providers perform PT tests to ensure your blood is clotting as it should. Clots that form too slowly can cause serious blood loss after an injury. Blood clots that form too fast can block blood vessels.
Your healthcare provider may perform a PT test to:
- Monitor how warfarin (Coumadin®) is working. Warfarin is a blood thinner that prevents dangerous blood clots. Without treatment, people at risk for blood clots can develop serious conditions such as deep venous thrombosis or pulmonary embolism. Providers most often do PT tests to ensure the warfarin dosage is right for you.
- Plan for surgery. Your provider may check that your blood is clotting normally before surgery to ensure you’re not at risk of losing too much blood.
- Diagnose bleeding and blood clotting disorders. PT tests help providers diagnose conditions that may be causing you to lose too much blood. Diagnosing these conditions can help explain symptoms like unexplained or frequent bruising, heavy nosebleeds or cuts that don’t stop bleeding.
- Check for a liver condition. Problems with prothrombin can signal liver disease. A PT test is one of several liver function tests providers use to determine how advanced liver disease is.
How does a prothrombin time test work?
A prothrombin time (PT) test is a simple blood test. After you give blood, a substance called thromboplastin gets added to activate clotting. Then, a lab specialist or a device calculates how long it takes for the sample to form a clot.
How do I prepare for a PT test?
Your healthcare provider will explain what you need to do to prepare. You may need to:
- Avoid certain medications. For example, certain medications (like some antibiotics) can increase your PT/INR results. Others, like oral birth control pills and hormone replacement therapy, can lower your PT time.
- Cut back on vitamin K. You may need to stop taking vitamin K supplements or cut back on foods high in Vitamin K, like green tea, asparagus or broccoli. Vitamin K can impact your results.
But check with your provider first. You should never stop taking your medications unless your provider says so.
What should I expect during a PT test?
The process is usually like giving blood. A healthcare provider will:
- Swab your skin with alcohol and place an elastic band around your upper arm.
- Ask you to make a fist to help the blood flow more easily.
- Insert a needle to collect blood into a vial. You may feel a sting when the needle goes in.
- Bandage the spot where they inserted the needle.
Sometimes, providers collect the sample by pricking your fingertip. There are also at-home PT test kits that use this method. You may need an at-home test if you’re on warfarin and need frequent checks. Your provider will instruct you on how to do the test and share your results.
What should I expect after a PT test?
You should be able to go about your regular routine after a PT test. You may have slight discomfort or bruising where the needle went in. But this doesn’t usually last long.
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