Primus Pathology Laboratory

Prothrombin Time and International Normalized Ratio (PT/INR) Test

Last Updated on July 28, 2022 by Dr. Neeraj Gujar

Overview of Prothrombin Time Test

Your body reacts quickly to a cut. To stop bleeding, cells called platelets first get there. Next, several proteins are found, which are called Clotting Factors. To stop bleeding, they all come together to form a blood clot.

This is what usually happens, however, if you are inclined to bleed easily or get clots where you shouldn’t then your clotting factors may be a problem.

This is when you may need a prothrombin time test to determine how fast your blood clots. This test is also known as a Prothrombin Time test (pro time) or an INR test.

Why Test Is Done?

Your body makes several different clotting factors. Any one of these factors can cause a problem that could affect the time it takes for a clot to form.

The Prothrombin Time test examines one of these factors to determine how effective they are.

This test is often performed in conjunction with the partial thromboplastin (PTT) test which examines another set of factors. They give your doctor a better picture of your body’s response to a clot.

When To Get Tested?

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
  • Gums that bleed easily
  • Blood in your poop or urine
  • Heavy menstrual periods in women
  • Nosebleeds
  • Swelling or pain in your joints

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

Also Read: What is the D-Dimer Test and Why Should You Care?

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

  • Immune system problems
  • Bone marrow problems
  • Lack of vitamin K, which is part of many clotting factors
  • Certain cancers, such as leukemia
  • Liver problems
  • Normal blood clotting before any surgery

How To Get Tested?

PT-INR Test Tube with blood sample

You don’t usually need much preparation. However, certain foods like liver, chickpeas, green tea, kale, turnip greens, or anything made from soy can harm the results. Your doctor will advise you if you should avoid certain foods and drinks.

Your doctor should be informed about any medications, vitamins, or herbs you are taking. This includes prescription and over-the-counter drugs. Your results can be affected by many medications. These include aspirin and steroids.

If you are taking warfarin, you should generally choose to take the test before you take your daily dose.

During the Test

This is a simple blood draw that takes only a few minutes. The lab tech will:

  • Clean the skin at the point where the needle goes in
  • Use a rubber band to wrap around your upper arm. This creates pressure that causes your veins to swell.
  • A thin needle should be inserted into a vein. This is usually located on the inside or back of your hand at your elbow.
  • Get the blood
  • Take off the rubber band and wrap your arm or hand in a bandage

Your doctor might use a fingerstick in some cases. This will provide you with faster results.

After The Test

You will feel a slight prick as the needle is inserted. This is usually the worst, but it’s possible to feel a slight prick when the needle goes in. Some side effects may include:

  • Bleeding and bruising
  • Infection
  • Feeling lightheaded or dizzy

Test Results

This test will tell you how long it took for your blood to form the clot. The normal ranges between different labs, so talk with your doctor about what your numbers mean.

While you may receive the results within a few hours or less, some labs can take up to several days. You can have results within minutes if your doctor uses a fingerstick.

The typical PT test result takes between 10 and 14 seconds. A PT result that is higher than this means your blood takes longer to clot. This could be an indication of:

  • Bleeding or clotting disorder
  • Inadequacy of clotting factors
  • Vitamin K deficiency
  • Liver Disease

A lower number can cause blood to clot faster than normal. It also may be a result of you taking vitamin K supplements or eating foods high in Vitamin K.

Also Read: What’s the Purpose of Liver Function Test? What Diseases Does It Detect?


What do high INR and PT mean?

An elevated INR or PT means that your blood takes longer to clot than is healthy for you. When your PT or INR levels are too high, you have an increased risk of bleeding.

What causes high prothrombin time?

Blood that clots too quickly is usually caused by supplements that are higher in vitamin K. High intake of foods containing vitamin K like broccoli, liver, green tea, kale, chickpeas, turnip greens, and soybean products.

What foods cause high INR levels?

Some of the foods that may affect the INR values are spinach, mustard greens, brussels sprouts, broccoli, asparagus, and other products high in Vitamin K.

by Dr. Neeraj Gujar

Dr. Neeraj Gujar has done his M.D. in Pathology from Government Medical College. He has worked at various organisations such as Breach Candy Hospital and Tata Memorial Hospital along with many other private organisations. During his working career, he realised the pressing need for exceptional quality diagnostic services. A diagnostic centre that can consistently give accurate results verified by a consultant doctor (M.D. Pathologist) on which your doctor can depend. This has been the founding principle of Primus Pathology and Diagnostics.

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