Prothrombin Time Test

A prothrombin time (PT) test measures how many seconds it takes for your blood to clot. Sometimes, this value gets expressed as the International Normalized Ratio (INR) instead of in seconds. The INR is a standardized measurement that accounts for differences in how labs calculate blood clotting time.

Healthcare providers do this test to see if your blood is clotting as it should. They may use it to:

• Make sure your warfarin dosage is right: You’re most likely to get this test if you’re on the blood thinner warfarin (Coumadin®). This medicine prevents serious blood clots.
• Check your liver: Your liver makes prothrombin. Prothrombin is one of many proteins called clotting factors that help form blood clots. An abnormal PT or INR may mean liver disease.
• Plan for surgery: Your provider may check that your blood is clotting normally before surgery to be sure there’s no risk of serious blood loss.
• Diagnose bleeding or blood-clotting disorders: Providers may do a PT test alongside a partial thromboplastin test to check how clotting factors are working. This helps diagnose conditions.

How does a prothrombin time test work?

A healthcare provider will remove a sample of your blood from a vein in your arm or from your fingertip. A substance called thromboplastin will be added to activate clotting. Then, a lab specialist or a device will calculate how long it takes for the sample to clot.

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

​What happens during the test?

The test is a simple blood draw. 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 thin needle to collect blood into a vial.
4. Bandage the spot where they inserted the needle.

At-home test kits use a fingertip prick to get the sample. You may need an at-home test if you’re on warfarin and need frequent checks. Your provider will explain how to do the test and share your results.

What happens after the test

Afterward, you’ll be able to go about your day as normal. Some people have slight bruising or discomfort where the needle went in. But this doesn’t usually last long.

What do test results look like?

Your prothrombin time is how many seconds it takes for your blood to clot. INR results give a number that reflects a ratio. Normal PT/INR ranges are:

• PT: 11 to 13.5 seconds
• INR if you’re taking warfarin: 2.0 to 3.0
• INR if you’re not taking warfarin: .8 to 1.1

But keep in mind that there are some conditions where a lower or higher range is actually a good thing. Your healthcare provider will let you know your target range.

Abnormal results

If you’re taking warfarin and your results are out of the therapeutic range, it may mean your healthcare provider needs to adjust your dosage. Sometimes, it means you’ve consumed medicine or food that interacts with warfarin. Your provider will tell you what can skew your results so you know to avoid them before your test.

If you’re not taking warfarin, a high or low PT can indicate different conditions or situations.

High PT/INR Levels

Your blood is clotting too slowly. This may mean you’re at increased risk of bleeding. Causes might be:

• An issue with your liver
• A vitamin K deficiency
• A bleeding disorder

Low PT/INR Levels

Your blood is clotting too fast. Causes include:

• Having too much vitamin K (either from the foods you eat or supplements)
• A blood-clotting disorder

Follow-up

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 other tests or procedures before your healthcare provider can make a diagnosis. They’ll explain the next steps.

If you’re taking warfarin and get abnormal results, your provider may adjust your dosage so your results are in the proper therapeutic range.

When should I call my doctor?

Reach out if you have any questions about how to prepare for the test or interpret your results. Remember, everyone’s situation is different based on their reasons for having the test.