About the INR
The International Normalized Ratio (INR) is the standard way to report a prothrombin time (PT) blood clotting test so that results are comparable across different laboratories and reagents. It is most often used to monitor people taking warfarin or a related vitamin K antagonist, where the goal is to keep blood clotting slowed to a safe, effective degree.
How the INR is calculated
A lab measures the patient's prothrombin time (PT) in seconds and compares it with the mean normal PT (the "control" value) for that laboratory's instrument and reagent combination. Because different thromboplastin reagents respond differently to warfarin, each reagent batch is assigned an International Sensitivity Index (ISI) that corrects for that difference. The INR formula is:
- INR = (patient PT / mean normal PT) ^ ISI
- PT ratio: the patient's PT divided by the control PT, before the ISI exponent is applied.
- ISI: typically close to 1.0 for modern, highly sensitive reagents, and higher for older or less sensitive ones.
Putting the result in context
Without anticoagulant therapy, INR normally sits around 0.8 to 1.1. On warfarin, most indications - atrial fibrillation, treatment of deep vein thrombosis or pulmonary embolism - target an INR of 2.0 to 3.0. Mechanical heart valves and some other high-risk situations use a higher target of 2.5 to 3.5. An INR below target suggests reduced protection against clotting, while an INR above target suggests an increased bleeding risk; the further from target, the more that risk changes.
When to consult a professional
This tool performs the standard INR arithmetic for education and record-keeping. It does not adjust medication doses or replace laboratory reporting. Any change to an anticoagulant dose or regimen should be made by the prescribing clinician, who weighs the INR alongside bleeding history, interacting medications, diet, and other individual factors.