How to Calculate INR

Prothrombin time is used to measure the blood coagulation in patients undergoing anticoagulation therapy (e.g. warfarin treatment). Briefly, the PT laboratory test includes mixing a commercially available reagent thromboplastin with the patient’s blood plasma and measuring the sample clotting time (usually in 10 to 20 seconds). However, PT time shows great variations, depending on a type of reagent and instrumentation. To standardize reported PT results, the World Health Organization has introduced International Normalized Ratio (INR). INR=(PT/MNPT)^ISI. PT is the patient’s prothrombin time 1. MNPT is the mean normal prothrombin time 1. ISI is the International Sensitivity Index determined for each batch of thromboplastin reagents by manufactures.

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Obtain the patient’s PT from records or elsewhere. Example: PT=15.2 s.

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Obtain the mean normal prothrombin time (MNPT) 1. It's usually reported by a laboratory based on PT tests (with a particular thromboplastin) on blood plasma samples from 20 to 30 apparently healthy people. For example, Tripodi and co-authors (see References, below) reported an MNPT of 13.1 s for thromboplastin Neoplastin plus.

Obtain the International Sensitivity Index (ISI) for the thromboplastin used. It's usually specified in the reagent certificate. An example of ISI for thromboplastin is 1.297.

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Calculate the International Normalized Ratio using the numbers above as an example 1. INR=(patient PT/MNPT)^ ISI = (15.2 s/13.1 s)^1.297=1.213. Note: Normal INR should be between 0.8 and 1.3 Low values of INR (less than 0.5 ) indicate a high risk of a clot, while high values (3 to 5) are associated with risk of bleeding.