Blood
International Normalized Ratio (INR)
Also known as: Prothrombin Ratio
Reference ranges by source
Published adult intervals: 3, from 3 named sources, shown in ratio.
About International Normalized Ratio (INR)
A prothrombin time (PT) test measures how many seconds it takes for a clot to form in a blood sample. Since the results of this test can vary with different labs or test methods, your results may be used to create an INR (international normalized ratio). Using this ratio makes it easier for health care providers to compare and better understand results from different labs or test methods.
Prothrombin is a protein made by the liver. It is one of several substances known as clotting (coagulation) factors. When you get a cut or other injury that causes bleeding, your clotting factors work together to form a blood clot. Blood clotting is an important process that prevents you from losing too much blood when you are injured. Clotting factor levels that are too low can cause you to bleed too much after an injury. Levels that are too high can cause dangerous clots to form in your arteries or veins.
A PT/INR test helps find out if your blood is clotting normally. It also checks to see if a medicine that prevents blood clots is working the way it should.
Main source: MedlinePlus ↗
Useful for
See how well warfarin, an anti-coagulant or blood-thinning medicine, is working to prevent blood clots. (Coumadin is a common brand name for warfarin.) Your provider may change your dose based on these results.
Find out the reason for abnormal blood clots.
Find out the reason for unusual bleeding.
Check clotting ability before a procedure or surgery that may cause bleeding.
Check for liver problems.
Check for vitamin K deficiency. Vitamin K helps with blood clotting.
Main source: MedlinePlus ↗
Interpretation
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.
If your INR is not normal, your provider will probably change your dose of warfarin.
If you are not taking warfarin, your results may be in the form of a PT time, which is the number of seconds it takes for your blood sample to clot (prothrombin time).
If you are not taking warfarin and your INR or prothrombin time results are not normal, it may be a sign that your blood is not clotting normally.
For general wellness information only. Talk to a clinician about your specific results.
Main source: MedlinePlus ↗
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Sources
Last updated 2026-05-02
This page presents source-specific reference data and sourced explanations. For general wellness information only. This is not medical advice. For diagnosis or treatment of any condition, talk to a qualified clinician.