Prothrombin Time Test

What is a prothrombin time (PT/INR) test?

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:

1. Swab your skin with alcohol and place an elastic band around your upper arm.
2. Ask you to make a fist to help the blood flow more easily.
3. Insert a needle to collect blood into a vial. You may feel a sting when the needle goes in.
4. 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.

What type of results do you get and what do the results mean?

Labs measure prothrombin time in seconds based on how long it takes your blood to clot, or (more commonly) as the International Normalized Ratio (INR). The World Health Organization (WHO) developed the INR as a standardized measurement that accounts for differences in lab testing processes.

The normal ranges for PT/INR results are:

• 11 to 13.5 seconds.
• INR of 0.8 to 1.1.
• INR of 2.0 to 3.0 if you’re taking warfarin. (There are some conditions for which a higher or lower target PT range is desired.)

Abnormal PT results

A high PT test result or INR level means it takes more time than usual for your blood to clot. If you’re not taking warfarin, it could be a sign of a bleeding or blood clotting disorder or a problem with your liver. A high result could also mean you’re not getting enough vitamin K.

If you’re taking warfarin, you may have an abnormal INR level (too high or low) if:

• You haven’t been taking the proper dose of warfarin.
• You’ve taken over-the-counter medicines, such as cold medicines, or vitamin supplements, that can interact with warfarin.
• You’ve consumed food and drinks that can interact with warfarin, like kale, spinach, cranberry juice and alcohol.
• You’re taking a prescription medicine that either increases or decreases the INR.

When should I know the results of the test?

Most people get their PT/INR test results within a few hours to one day. Finger prick tests can produce results in minutes. If your provider has to send the sample to an off-site lab for testing, you may have to wait a few days.

If the results are abnormal, what are the next steps?

Providers usually consider the results of the PT test and the PTT test together to determine what next steps to take. Depending on your situation, your healthcare provider may order additional tests so they can diagnose and treat the problem. A PT test is usually one of the first tests you’ll take to diagnose a bleeding or blood clotting disorder. You’ll likely need others before your provider can make a diagnosis.

If you’re taking warfarin, they may adjust your dosage so your blood is clotting as it should without putting you at risk of developing blood clots.

When should I call my healthcare provider?

Ask your provider if you have any questions about how to prepare for a PT test, how you’ll receive results or what your next steps will be. Everyone’s situation is different. Your provider can guide you on what your results mean in terms of your care plan going forward.