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INR Calculator

Estimate INR from patient prothrombin time, control PT, and reagent ISI with the WHO INR formula. Includes assumptions, common limits, examples, and a medical disclaimer.

seconds

Prothrombin time in seconds from the patient's test.

seconds

The laboratory's normal or control PT in seconds.

Common reagent ISI range is 0.8 to 3.0.

Common use limits

This calculator uses the WHO INR formula. It is commonly used only when the reagent ISI is between 0.8 and 3.0 and both PT values are in seconds.

The output is an educational estimate. It does not replace a lab-reported INR or tell you how to adjust anticoagulant medication.

This INR calculator provides educational arithmetic and is not a substitute for a laboratory-reported INR, medical advice, diagnosis, or anticoagulation management. Never start, stop, skip, double, or change warfarin or another medicine based on this output. Contact the prescribing or anticoagulation team about unexpected results, missed or extra doses, illness, diet or medicine changes, bleeding, bruising, or procedures. Seek emergency help for uncontrolled or severe bleeding, head injury, stroke symptoms, chest pain, breathing trouble, collapse, or another suspected emergency.

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