Prothrombin time test

The prothrombin time test, sometimes referred to as the PT or pro time test, checks how quickly blood is clotting.

Prothrombin is a protein produced by the liver. It is one of many factors in the blood that help it to clot properly.

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

A healthcare professional 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 done 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, also called MELD — is a scoring system for assessing the severity of chronic liver disease.

If your health professional suspects that you have other health concerns, such as a liver condition 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

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

After the test

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

In some clinics, a medical professional takes 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 above that range means it takes blood longer than usual to clot. A number below that range means blood clots more quickly than usual.

As INR

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

In people without health conditions, an INR of 1.1 or below is considered typical. An INR range of 2.0 to 3.0 is generally an effective therapeutic range for people taking warfarin for certain conditions. These conditions include atrial fibrillation or a blood clot in the leg or lung. In certain situations, such as having a mechanical heart valve, a slightly higher INR may be needed.

When the INR is above the recommended range, it means that the blood clots more slowly than desired. A lower INR means the 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 conditions.
• Not enough of the proteins that cause blood to clot.
• Not enough vitamin K.
• Other substances in the 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 medicines, such as birth control pills and hormone replacement therapy.