Prothrombin Time Test and INR (PT/INR) Test
A PT/INR test measures how long it takes for your blood to clot. It helps check if your blood is clotting normally or if a blood-thinning medicine like warfarin is working correctly.
What this test measures
The PT test measures the number of seconds it takes for a clot to form in a blood sample. The INR is a standardized ratio that allows comparison of results from different labs. Prothrombin is a clotting protein made by the liver.
What your results mean
Reference ranges vary by laboratory and by your age and sex. MedlinePlus does not publish a single normal range for this test — always read your result against the range printed on your own lab report and discuss it with your provider.
Higher-than-normal results
INR levels that are too high may mean you are at risk for dangerous bleeding because your blood is taking longer than normal to form a clot. This may be due to a bleeding disorder, liver disease, or vitamin K deficiency.
Lower-than-normal results
INR levels that are too low may mean you are at risk for dangerous blood clots because your blood clots too easily. This may be due to a clotting disorder, supplements with vitamin K, a diet high in vitamin K, or certain medicines.
Frequently asked questions
What is the difference between PT and INR?
PT is the time in seconds for a clot to form. INR is a standardized ratio that makes it easier to compare results from different labs or test methods.
Why is this test used for people on warfarin?
The test checks how well warfarin is working to prevent blood clots. Your provider may change your dose based on the results.
What can cause high PT/INR levels?
High levels may be due to a bleeding disorder, liver disease, or vitamin K deficiency.
What can cause low PT/INR levels?
Low levels may be due to a clotting disorder, supplements with vitamin K, a diet high in vitamin K, or certain medicines.
Can food or alcohol affect my PT/INR results?
Yes. Foods high in vitamin K (like green leafy vegetables), soybean and canola oils, high-fat diets, and alcohol can affect your results.
Related lab tests
Sources
- MedlinePlus: Prothrombin Time Test and INR (PT/INR) — NIH MedlinePlus
Last updated . Information is aggregated from official public sources and is not a substitute for professional medical care.