Most often, the prothrombin time is monitored if you are taking the blood-thinning medication warfarin. In this situation, the prothrombin time is shown as an international normalized ratio (INR).

Your health care provider may recommend a prothrombin time test before surgery if there is any concern about your blood's ability to clot.

The prothrombin time test also may be performed to check for liver disease. It is one of many tests used to screen people waiting for liver transplants. That screening — known as the model for end-stage liver disease (MELD) — is a scoring system for assessing the severity of chronic liver disease.

If your provider suspects you have other health issues, such as liver problems or a bleeding disorder, you might need more testing to confirm your condition.

A prothrombin time test is similar to any other blood test. You might experience soreness or minor bruising at the site in the arm where your blood is drawn.

During the test

Obtaining the blood sample for prothrombin time testing usually takes just a few minutes, like any other blood test.

After the test

Your blood might be sent to a laboratory for analysis. If the laboratory analysis is done on-site, you could have your test results within hours. If your provider sends your blood to an off-site laboratory, it may take several days to receive the results.

In some clinics, a nurse will take a sample of your blood with a finger stick. You may receive the test result within minutes, while you are still in the exam room.

Home testing

Home testing kits are available for people who have to take blood thinners for long periods and who have been trained in taking blood samples and testing them.

Prothrombin time test results can be presented in two ways.

In seconds

The average time range for blood to clot is about 10 to 13 seconds. A number higher than that range means it takes blood longer than usual to clot. A number lower than that range means blood clots more quickly than normal.

As INR

This ratio — which allows for easier comparisons of test results from different laboratories — is used if you take blood-thinning medications.

In healthy people an INR of 1.1 or below is considered normal. An INR range of 2.0 to 3.0 is generally an effective therapeutic range for people taking warfarin for certain disorders. These disorders include atrial fibrillation or a blood clot in the leg or lung. In certain situations, such as having a mechanical heart valve, you might need a slightly higher INR.

When the INR is higher than the recommended range, it means that your blood clots more slowly than desired. A lower INR means your blood clots more quickly than desired.

What your results mean

Clotting too slowly

Blood that clots too slowly can be caused by:

• Blood-thinning medicines
• Liver problems
• Inadequate levels of proteins that cause blood to clot
• Vitamin K deficiency
• Other substances in your blood that prevent the work of clotting factors

Clotting too fast

Blood that clots too quickly can be caused by:

• Supplements that contain vitamin K
• High intake of foods that contain vitamin K, such as liver, broccoli, chickpeas, green tea, kale, turnip greens and products that contain soybeans
• Estrogen-containing medications, such as birth control pills and hormone replacement therapy