Prothrombin Time and INR (PT/INR)
Reviewed
Also called: Prothrombin time, International normalised ratio, INR, Pro time
PT measures clotting time, while INR standardises PT reporting to improve comparison between laboratories and methods.
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
Tell the clinician about anticoagulants, other medicines, supplements, recent illness, and diet changes. Never change warfarin or another anticoagulant based only on this page.
A venous sample or, in some monitoring settings, a finger-prick sample may be used.
Blood collection has a small risk of brief discomfort, bruising, light-headedness, or rarely infection at the needle site.
Understanding results
Meaning depends on why the test was ordered. A treatment target for a person taking warfarin is different from an interval used when not taking warfarin.
- Above the intended range: Blood is taking longer to clot than intended for that clinical situation. Limitation: Cause and urgency depend on the value, bleeding, warfarin indication, medicines, illness, liver function, and recent changes. Next step: Contact the anticoagulation or treating team promptly; urgent assessment is needed when bleeding or severe symptoms occur.
- Below the intended range: For warfarin monitoring, anticoagulation may be less than intended; outside treatment, interpretation differs. Limitation: Do not change a dose without the responsible clinician's protocol. Next step: Report the result to the monitoring team and follow their instructions.
- 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
The responsible team may repeat testing, assess bleeding or clotting risk, review interactions, and adjust treatment according to a formal protocol.
- 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
U.S. National Library of Medicine: Prothrombin Time and INR (PT/INR)