Hepatology and coagulation
PT to INR
The International Normalized Ratio standardizes a prothrombin time so results are comparable between laboratories using different reagents. It is what warfarin is monitored against.
Enter the values
The International Sensitivity Index printed on the thromboplastin reagent.
The result appears as you type.
Nothing is sent anywhere. The calculation runs entirely in your browser.
This calculator is provided for reference and education. It does not replace clinical assessment, examination, current guidelines, local protocol, or professional judgment. Verify every result before acting on it. Nirogix accepts no liability for clinical decisions made using this tool.
The formula
- INR = (patient PT ÷ mean normal PT) ^ ISI
Worked example
PT 24 s, control 12 s, ISI 1.2 → (2.0)^1.2 = 2.30
When to use it
- Reporting or checking an INR where the laboratory value is unavailable
- Understanding how a reagent’s ISI affects the result
And when not to
- INR is standardized for warfarin monitoring. It is not a reliable measure of bleeding risk in liver disease, where it is often misread as one.
- It does not monitor direct oral anticoagulants and must never be used to.
- A point-of-care meter and a laboratory analyzer can disagree, particularly at high values.
References
- Guidelines on oral anticoagulation with warfarin — British Committee for Standards in Haematology, British Journal of Haematology, 2011
Last reviewed 07/09/2026.
These calculators are free and always will be. We also build Nirogix, a hospital management system for Indian clinics and hospitals. The same calculations are planned inside the consultation record, pre-filled from the chart.