INR Calculator: International Normalised Ratio

The International Normalised Ratio (INR) is a standardised way to measure blood clotting speed, derived from the Prothrombin Time (PT) test. It was introduced by the WHO in 1983 to make PT results comparable across different laboratories and reagents. INR is primarily used to monitor the anticoagulant effect of warfarin (Coumadin) and to screen for coagulation disorders. A normal INR for a person not on anticoagulants is approximately 0.8-1.2.

For most indications, the target INR range on warfarin is 2.0-3.0. This includes atrial fibrillation, deep vein thrombosis (DVT), pulmonary embolism (PE), and most mechanical heart valve types. For patients with certain high-risk mechanical heart valves (older ball or disc valves in the mitral position), the target is 2.5-3.5. INR below 2.0 risks thrombosis; INR above 3.0-4.0 significantly increases bleeding risk.

The ISI is a calibration factor assigned to each batch of thromboplastin reagent used in the PT test. Different laboratories use different reagents, and the ISI allows results to be standardised to a reference WHO thromboplastin. The ISI is provided by the reagent manufacturer and is typically 1.0-2.0 for modern reagents. The formula INR = (PT patient / PT normal)^ISI converts raw PT ratios to standardised INR values.

Many factors alter INR in patients on warfarin: foods high in vitamin K (leafy greens like kale, spinach, broccoli) lower INR; many medications interact with warfarin (antibiotics, NSAIDs, antifungals); alcohol, liver disease, heart failure, diarrhoea, and fever can raise INR. Warfarin has a narrow therapeutic window, which is why regular monitoring (typically every 4 weeks once stable) is essential.

INR = (Patient PT / Mean Normal PT)^ISI. The mean normal PT is the geometric mean PT from at least 20 healthy adults using the same reagent lot, typically provided by the lab as a reference value (often 11-13 seconds for modern reagents). The ISI is the International Sensitivity Index for the specific reagent used. Most labs now report INR directly, but this formula is useful for understanding how the calculation works.