INR Calculator (from PT and ISI)
INR Calculator (from PT and ISI)
Convert a patient's prothrombin time into an INR using the laboratory's mean normal PT and the reagent's ISI — the calculation that makes PT results comparable between laboratories and reagents.
INR (from PT and ISI)
PT ratio ^ ISIPT 22 s, mean normal PT 12.5 s, ISI 1.05
Formula
- PT
- patient prothrombin time, seconds
- MNPT
- the reporting laboratory's own mean normal prothrombin time, not a textbook constant
- ISI
- International Sensitivity Index of the thromboplastin reagent — closer to 1.0 means a more responsive, more precise reagent
Worked example
PT 22 s, mean normal PT 12.5 s, ISI 1.05
22 ÷ 12.5 = 1.76
1.76^1.05 = 1.81
1.81 is below 2.0 — sub-therapeutic for standard warfarin indications
Typical target INR ranges
| Indication | Target range |
|---|---|
| Atrial fibrillation | 2.0 – 3.0 |
| VTE treatment or secondary prevention | 2.0 – 3.0 |
| Mechanical aortic valve | 2.0 – 3.0 |
| Mechanical mitral valve | 2.5 – 3.5 |
| Recurrent VTE on therapeutic warfarin | 3.0 – 4.0 |
Why the INR exists, and where it stops applying
Different thromboplastin reagents produce different prothrombin times for the same plasma, which made raw PT results useless for comparing a patient’s anticoagulation between laboratories or over time. The International Sensitivity Index calibrates each manufacturer’s reagent against a World Health Organization international reference thromboplastin, and raising the PT ratio to the power of the ISI corrects for that reagent’s particular responsiveness. A reagent with an ISI close to 1.0 behaves like the reference material and gives the most precise, most reproducible INR; a high-ISI reagent amplifies any measurement error in the underlying PT.
The INR was validated for one purpose: monitoring stable dosing of a vitamin K antagonist such as warfarin. Within that use it is genuinely interchangeable between laboratories, which is the entire point of the exercise.
Outside that use it is not a valid measure of coagulopathy. In liver disease, disseminated intravascular coagulation, or in a patient taking a direct oral anticoagulant, the PT is prolonged by a different and non-standardised mechanism, and the ISI correction was never calibrated for it — the resulting number is not a true INR even though it is reported as one. It is nevertheless used, by long-standing convention rather than validation, inside composite scores such as MELD and Child-Pugh, where clinicians should recognise it is being repurposed rather than measuring what it was designed to measure.
Frequently asked questions
What is a normal INR?
Around 0.8 to 1.2 in a person not taking a vitamin K antagonist. Therapeutic ranges on warfarin are higher and depend on the indication — typically 2.0 to 3.0.
Why is the INR not valid in liver disease?
The INR was calibrated and validated only for monitoring stable vitamin K antagonist therapy. In liver disease the PT is prolonged by reduced synthesis of multiple factors, a different mechanism the ISI correction was never designed for, so the resulting number does not carry the same meaning.
What does the ISI actually correct for?
Different thromboplastin reagents are more or less sensitive to reduced clotting factors, so the same plasma gives different PT results on different reagents. The ISI, derived against a WHO reference thromboplastin, corrects for that reagent-specific sensitivity.
If INR isn't valid in liver disease, why does MELD use it?
By convention rather than validation — MELD was derived using the INR as an available, reproducible marker of synthetic function, not because the INR retains its calibrated meaning in that setting.
Related calculators
References
- Kirkwood TB. Calibration of reference thromboplastins and standardisation of the prothrombin time ratio. Thromb Haemost. 1983;49(3):238–44.
- Ansell J et al. Pharmacology and management of the vitamin K antagonists: American College of Chest Physicians evidence-based clinical practice guidelines (8th edition). Chest. 2008;133(6 suppl):160S–198S.