Prothrombin Time and INR (PT/INR)
Reviewed
Also called: Prothrombin time, International normalised ratio, INR, Pro time
A prothrombin time (PT) test measures how many seconds it takes for a clot to form in a blood sample.
Specimen: Blood
Urgent safety information
Seek urgent care for major or uncontrolled bleeding, vomiting blood, black stools, sudden severe headache, collapse, chest pain, or sudden breathing difficulty regardless of the latest INR.
Why this test may be ordered
- Monitoring warfarin treatment
- Investigating unusual bleeding or clotting
- Assessing clotting before a procedure or in liver/vitamin K problems
Preparation and collection
If you are taking warfarin, you may need to delay your daily dose until after testing. Your provider will let you know if there are any other special instructions to follow.
The test may be done on a blood sample from a vein or a fingertip. A health care professional will take a blood sample from a vein in your arm, using a small needle. A fingertip test may be done in a provider's office or in your home. If you are using an at-home test kit, you will need to review your results with your provider.
There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
Understanding results
If you were tested because you are taking warfarin or have certain health conditions, your results will probably be in the form of INR levels.
- INR too high (taking warfarin): 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. Limitation: Be sure to tell your provider if you are taking any medicines or supplements. Some may affect your results. Next step: If your INR is not normal, your provider will probably change your dose of warfarin.
- INR too low (taking warfarin): INR levels that are too low may mean you are at risk for dangerous blood clots because your blood clots too easily. Limitation: Other things that may affect your PT/INR results include: Alcohol Foods high in vitamin K, such as green leafy vegetables Soybean and canola oils High-fat diets Next step: If your INR is not normal, your provider will probably change your dose of warfarin.
- PT/INR too high (not taking warfarin): If your PT/INR levels are too high, your blood is clotting too slowly. This may be due to: A bleeding disorder, a condition in which your body can't clot blood well. Limitation: Be sure to tell your provider if you are taking any medicines or supplements. Some may affect your results. Next step: To understand the results of a PT/INR test, your provider will consider your symptoms, medical history, and the results of other blood tests.
- PT/INR too low (not taking warfarin): If your PT/INR levels are too low, your blood is clotting too quickly. This may be due to: A clotting disorder, a condition in which your body forms too many clots in arteries or veins. Limitation: Be sure to tell your provider if you are taking any medicines or supplements. Some may affect your results. Next step: To understand the results of a PT/INR test, your provider will consider your symptoms, medical history, and the results of other blood tests.
- INR targets are indication-specific and patient-specific.
- Medicines, antibiotics, illness, alcohol, diet, supplements, liver disease, and collection problems can affect results.
What to do next
To understand the results of a PT/INR test, your provider will consider your symptoms, medical history, and the results of other blood tests.
- Which reference interval and units apply to my result?
- Could medicines, supplements, illness, or sample timing affect this result?
- Do I need a repeat test or another test before any conclusion is made?
Evidence source
Source: U.S. National Library of Medicine (MedlinePlus). Practices and reference ranges may differ in your country.
U.S. National Library of Medicine: Prothrombin Time and INR (PT/INR)