Prothrombin Time / INR (PT/INR)
Prothrombin time (PT) measures how quickly plasma clots after adding tissue factor. INR (International Normalized Ratio) standardizes the PT result across different labs so warfarin dosing is consistent worldwide. PT/INR is used to monitor warfarin therapy, evaluate liver function, and diagnose bleeding disorders involving the extrinsic coagulation pathway.
Reference Ranges
Male
0.9 – 1.2
INR
Female
0.9 – 1.2
INR
Child
0.9 – 1.2
INR
Reference ranges may vary by laboratory. Always compare with the range printed on your lab report.
What Low Levels Mean
An INR below 0.9 is unusual and may reflect laboratory error, very recent vitamin K intake, or — in a patient on warfarin — undertreatment and increased clot risk. In otherwise healthy people, a low INR has no clinical significance on its own.
What High Levels Mean
A high INR means slower clotting and a higher bleeding risk. In warfarin patients the target INR is typically 2.0–3.0 (higher for mechanical heart valves). Outside of warfarin therapy, high INR suggests liver disease, vitamin K deficiency, DIC, or inherited factor deficiency. INR above 5 with warfarin is a bleeding emergency.
Frequently Asked Questions
What is the difference between PT and INR?
PT is the raw clotting time in seconds — but different labs and different reagents produce different PT values, so results cannot be compared across labs. INR is a mathematically standardized version of PT that corrects for these differences. Always use INR, not PT in seconds, for warfarin dosing and trending.
What is the target INR on warfarin?
For most indications (atrial fibrillation, DVT/PE, biological heart valves) the target is 2.0–3.0. For mechanical mitral valves, target is 2.5–3.5. Newer direct oral anticoagulants (DOACs) like apixaban and rivaroxaban do not require INR monitoring and are increasingly preferred.
Can diet affect my INR?
Yes. Vitamin K — found in green leafy vegetables (spinach, methi, kale) — reverses the effect of warfarin. You do not have to avoid these foods, but keep your intake consistent from week to week. Large swings in vitamin K intake cause large swings in INR. Newer DOAC anticoagulants are not affected by vitamin K.
Related Coagulation tests
See all →This information is for educational purposes only and should not replace professional medical advice. Always consult your doctor for interpretation of your test results.
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