INR Calculator — International Normalized Ratio

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How this is calculated

The International Normalized Ratio (INR) was introduced by the WHO in 1983 to standardise PT-based anticoagulation monitoring across laboratories that use different thromboplastin reagents. Different reagents respond differently to clotting-factor deficiencies, so a raw PT in seconds is not directly comparable between labs. The INR corrects for this by raising the PT ratio (patient PT divided by the lab's mean normal PT) to the power of the reagent's International Sensitivity Index (ISI), published on the reagent package insert. An ISI of 1.0 means the reagent is perfectly calibrated to the WHO reference thromboplastin.

For healthy people not on anticoagulants, the normal INR range is 0.8–1.2. Patients taking warfarin (or other vitamin K antagonists) for conditions like atrial fibrillation, deep vein thrombosis, pulmonary embolism, or bioprosthetic heart valves are typically targeted at an INR of 2.0–3.0. Patients with mechanical heart valves may have a higher target of 2.5–3.5. An INR above 4–5 indicates a significantly elevated bleeding risk and usually requires dose adjustment or reversal.

Important caveats: the mean normal PT must come from the specific reagent lot used in the patient's test — the laboratory's computer normally performs this calculation automatically. If your lab already reports the INR directly on the test result slip, you do not need to recalculate it. This calculator is a cross-check and educational tool, not a substitute for laboratory-reported INR or clinical guidance.