Prothrombin time (PT/INR)
Quick answer: The prothrombin time measures how long your blood takes to clot, and is often reported as an INR. It is used to monitor warfarin and similar blood thinners, check liver function, and screen for bleeding problems before surgery.
- SampleBlood
- TakesAbout 5 minutes
- Before the testNo special preparation is usually needed; tell staff about any blood thinners you take, and do not change your dose without your doctor's advice.
How the test is done
- 1Blood is drawn into a light-blue citrate tube
- 2You tell the staff if you take warfarin or another blood thinner
- 3The lab adds a reagent to your plasma
- 4The clotting time is measured and converted to an INR
What your results can mean
Reading your report
Target INR ranges depend on why you are being tested; discuss your result with your doctor.
If the result is outside the usual range
A long PT or high INR can be seen with warfarin, liver disease, vitamin K deficiency or some clotting factor problems. In people on warfarin, the INR shows whether the dose is in the target range set by their doctor.
Who usually has this test
People on warfarin, with liver disease, before surgery, or with unusual bleeding.
Risks and aftercare
Bruising is possible; press for longer if you are on blood thinners.
Names and options
Also called: PT, INR, PT INR, pro time
Listed by the hospital as
- PROTHROMBIN TIME
Questions about this test
What is INR?
INR is a standard way of reporting PT so results can be compared between labs. It is mainly used for people on warfarin.
Can food affect my INR?
Foods high in vitamin K, such as green leafy vegetables, can lower the INR on warfarin. Keep your diet steady and tell your doctor about big changes.
Why is PT checked in liver disease?
The liver makes most clotting factors. A long PT can show how well the liver is working.
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This page explains each test in general terms so you know what to expect. It is not medical advice: your doctor decides which tests you need and what your results mean. Reference ranges differ between laboratories. Updated 4 November 2026.