A prothrombin time test measures how quickly your blood clots. Sometimes called a PT or pro time test, a prothrombin time test uses a sample of your blood.
Prothrombin is a protein produced by your liver. It is one of many factors in your blood that help it to clot appropriately.
If you take a blood-thinning medication such as warfarin (Coumadin), your prothrombin time test results will be expressed as a ratio called the international normalized ratio (INR).
Your doctor might recommend a prothrombin time test to:
- Monitor the effectiveness of a blood-thinning medication that you're taking
- Diagnose liver problems
- Assess your blood's ability to clot before you have surgery
The prothrombin time test is one of several 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.
When you have a prothrombin time test, you might also have:
- Additional testing to assess the health of your liver, if your doctor suspects you have liver problems
- Additional clotting-function tests, if your doctor suspects you have a bleeding disorder
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 doctor sends your blood to an off-site laboratory, it may take several days to receive the results.
In some anticoagulation clinics, a nurse will take a sample of your blood with a finger stick, and give you the test result within minutes, while you are still in the exam room.
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.
The average time range for blood to clot is about 10 to 14 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.
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 disorders such as 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, and 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 medications
- Liver problems
- Inadequate levels of proteins that cause blood to clot
- Vitamin K deficiency
- Other substances in your blood that hinder 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