Overview
The prothrombin time, sometimes referred to as PT or pro time test, is a test to evaluate blood clotting.
Prothrombin is a protein produced by your liver. It is one of many factors in your blood that help it to clot appropriately.
Why it's done
Most often, the prothrombin time is monitored if you are taking the blood-thinning medication warfarin. In this situation, the prothrombin time is expressed as an international normalized ratio (INR).
Your doctor may recommend a prothrombin time test prior to surgery if there is any concern about your blood's ability to clot.
The prothrombin time test may also be performed to evaluate you for liver disease. It 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.
If your doctor suspects you have other health issues, such as liver problems or a bleeding disorder, you might need additional testing to confirm your condition.
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Risks
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.
What you can expect
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
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.
Results
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 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
Dec. 08, 2020
After being diagnosed with a blood clot (thrombosis), your healthcare provider may prescribe warfarin for treatment. Warfarin is a medication that works by blocking the formation of vitamin-K-dependent clotting factors, which are substances in the blood that cause clotting.
Although warfarin can prevent blood clots from forming, it can also cause other problems like bleeding. To make sure that the medication is working effectively, patients on warfarin need routine blood tests to check a level called the INR.
What does INR stand for?
INR stands for international normalized ratio and is measured with a blood test called PT-INR. PT stands for prothrombin time. The test measures how much time it takes for your blood to clot and will determine if you’re receiving the right dose of warfarin. If your INR is too low, you could be at risk for a blood clot – but if it’s too high, you could experience bleeding.
A typical INR target ranges from 2-3 but can vary from patient to patient. Patients with a tendency towards clotting may have a range targeting 3-4, whereas patients with a higher bleeding risk may have a lower INR between 2-2.5.
How often is testing needed?
Testing needs depend on how stable a patient’s INR is over time. According to the American Heart Association (AHA), patients should be tested at least once a month, but some patients will require testing as often as twice a week. INR tests can be done at a lab or clinic but can also be done at home for some patients. Home testing requires the use of an in-home INR monitoring machine and test strips. After self-testing with this device, you can report your INR results to your care team to determine if you need to adjust your warfarin dose. Learn more about home testing here or talk to your healthcare provider.
What can I do to help keep my INR level where it should be?
First, always listen closely to your doctor’s instructions and take your medication exactly as prescribed. If you miss any doses, you should contact your healthcare provider and ask about how to safely restart it.
When filling your warfarin prescription, try to fill it at the same pharmacy to ensure the brand of warfarin you’re on stays consistent. You also need to make sure that your doctor always has an up-to-date list of all the medications you take since some medications may interact with warfarin.
Finally, you’ll also need to watch your diet and keep track of foods that may have vitamin K in them. Because warfarin works by blocking vitamin-K-dependent clotting factors, increasing your vitamin K intake can make warfarin less effective by lowering your INR level and potentially putting you at risk for a blood clot. Foods with high levels of vitamin K include kale, broccoli, and Brussels sprouts. Other changes to your nutrition, such as starting a new diet or taking new supplements, can also cause your INR level to fluctuate.
Talk to your doctor before starting any new diet/nutrition regimens.
Looking forward
Starting a new medication can be challenging and warfarin is no exception. If you have questions or concerns about taking warfarin, be sure to contact your healthcare provider. In addition, pharmacists at either your local hospital or retail pharmacy are an excellent resource. Sometimes your doctor may refer you to a specialized clinic called an anticoagulation management service. The medical staff in these clinics will check your INR and help make adjustments to your warfarin dose if needed.