Blood test
INR Test (Clotting)
An INR test measures how long your blood takes to clot. It is mainly used to monitor the blood-thinning medicine warfarin and keep the dose in a safe range.
Quick answer
INR Test (Clotting): what it is and what the results mean
The INR (international normalised ratio) is a standardised measure of how quickly the blood clots, based on the prothrombin time. It is most often used to monitor people taking the anticoagulant warfarin.
- Why it is done: It is used to check that warfarin is thinning the blood by the right amount — enough to prevent clots but not so much as to cause bleeding — and to investigate clotting or bleeding problems and liver function.
- Understanding results: A higher INR means the blood is taking longer to clot (thinner blood); a target range is set for each person on warfarin.
What it is
The INR (international normalised ratio) is a standardised measure of how quickly the blood clots, based on the prothrombin time. It is most often used to monitor people taking the anticoagulant warfarin.
Why it is done
It is used to check that warfarin is thinning the blood by the right amount — enough to prevent clots but not so much as to cause bleeding — and to investigate clotting or bleeding problems and liver function.
What to expect
A blood sample is taken from the arm, or sometimes from a fingerprick using a small monitor. People on warfarin have the test regularly, with the dose adjusted according to the result.
Understanding the results
A higher INR means the blood is taking longer to clot (thinner blood); a target range is set for each person on warfarin. Results outside the range prompt a dose change, and a very high INR may need action to reduce bleeding risk.
Good to know
Risks and limitations
Risks are those of a routine blood test. The INR is affected by diet, other medicines and illness, which is why regular monitoring is important for people on warfarin.
Education and reference only. This explains the test in general terms and is not medical advice. Always follow the specific instructions from the team arranging your test, and discuss your results with your clinician.
Answers
INR Test (Clotting): frequently asked questions
What is a normal INR?
For someone not on blood thinners, the INR is around 1. People taking warfarin have a target range (often about 2 to 3) set by their clinician depending on why they are treated.
What affects my INR?
Diet (especially vitamin-K–rich foods), alcohol, illness and many other medicines can change the INR. This is why the test is repeated regularly and the warfarin dose adjusted.
Related tests
Other blood test investigations
Sources
Where this is drawn from
- NHS — Tests and treatments
- NICE — diagnostic guidance
- Royal College of Pathologists / relevant professional body
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