What Is a Prothrombin Time Test?

When you get a cut, your body jumps into action to keep things from getting out of hand. Cells called platelets get there first to slow the bleeding. Then, a bunch of proteins, called clotting factors, show up. They all fit together to form a solid mass -- a blood clot -- to stop the bleeding so you can start healing.

That’s what typically happens. But if you tend to bleed easily or you get clots when you shouldn’t, then you may have a problem with your clotting factors.

That’s when you might need a prothrombin time test, which measures how quickly your blood clots. It’s also called a PT, pro time, or INR test.

What Does a PT Test Look For?

Your body makes several different clotting factors. A problem with any of them -- if it’s missing, broken, or you don’t have enough -- can affect how long it takes a clot to form, if it forms at all.

A PT tests looks at one set of these factors to see how well they’re working.

It’s often done along with another test, called the partial thromboplastin time (PTT) test, which looks at another set of factors. Together, they give your doctor a more complete picture of what happens in your body when a clot forms.

When Would I Need a PT Test?

Your doctor might order this test to check for a bleeding disorder. Symptoms of bleeding disorders include:

  • Bleeding or bruising easily
  • Blood clots that form when they shouldn’t
  • Blood in your poop or urine
  • Gums that bleed easily
  • Heavy menstrual periods in women
  • Nosebleeds
  • Swelling or pain in your joints

You’d also need this test if you’re on warfarin (Coumadin, Jantoven), a medicine to thin your blood. The PT test helps make sure you get the right dose -- you want to prevent dangerous clots, but still let your blood clot when you need it to.

Your doctor might also suggest you get this test to check for:

  • Bone marrow problems
  • Certain cancers, such as leukemia
  • Immune system problems
  • Lack of vitamin K, which is part of many clotting factors
  • Liver problems (your liver makes the clotting factors)
  • Normal blood clotting before you have surgery

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What Happens During the Test?

This test is a basic blood draw and takes just a few minutes. A lab tech will:

  • Clean the skin where the needle goes in
  • Wrap a rubber strap around your upper arm -- this creates pressure to make your veins swell with blood
  • Insert a thin needle into a vein, usually on the inside of your arm at your elbow or in the back of your hand
  • Draw the blood
  • Remove the rubber strap and put a bandage on your arm or hand

In some cases, your doctor may use a finger stick instead, which will give you results more quickly.

What Are the Risks?

Typically, you’ll feel a light prick when the needle goes in. That’s usually the worst of it, but since you’re having your blood drawn, there’s a slight chance of:

  • Bleeding or bruising
  • Feeling dizzy or lightheaded
  • Infection

How Do I Prepare?

Typically, you don’t need to do anything special. Some foods -- such as liver, broccoli, chickpeas, green tea, kale, turnip greens, and foods made from soy -- can affect your results. If you need to avoid certain foods or drinks, your doctor will tell you ahead of time.

Make sure to let your doctor know about any medicines, vitamins, herbs, or supplements you take. That includes over-the-counter, prescription, or illegal drugs. Many medications, from aspirin to steroids, can affect your results.

Also, if you’re on warfarin, you typically want to get the test before you take your dose for the day.

What Do the Results Mean?

The test tells you how many seconds it took your blood to form a clot. What’s normal varies with different labs, so check with your doctor to help you understand what your numbers mean.

You may get the results in just a few hours, but some labs could take several days. If your doctor uses a finger stick, you can get results in just a few minutes.

A typical PT result is 10 to 14 seconds. Higher than that means your blood is taking longer than normal to clot and may be a sign of many conditions, including:

  • Bleeding or clotting disorder
  • Lack of vitamin K
  • Lack of clotting factors
  • Liver disease

If you have a lower number, your blood clots faster than normal. This could be from taking supplements or eating foods high in vitamin K.

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Will I Get Any Other Tests?

It depends on what your doctor is looking for. For example, if your doctor thinks you might have a liver problem, you’ll likely get more tests to check how your liver’s working.

If your doctor thinks you have a bleeding disorder, you may get tests to check for those, such as:

  • PTT test
  • Platelet count
  • Thrombin time test
  • von Willebrand factor test (looks for a genetic disorder that causes nosebleeds and other bleeding problems)
WebMD Medical Reference Reviewed by Minesh Khatri, MD on January 18, 2017

Sources

SOURCES:

KidsHealth: “Blood Test: Prothrombin Time,” “Blood Test: Partial Thromboplastin Time (PTT).”

University of Rochester Medical School: “Prothrombin Time.”

Lab Tests Online: “Prothrombin Time and International Normalized Ratio.”

Mayo Clinic: “Prothrombin Time Test.”

National Hemophilia Foundation: “Von Willebrand Disease.”

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