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How to Calculate INR

By Contributing Writer ; Updated July 27, 2017

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. MNPT is the mean normal prothrombin time. ISI is the International Sensitivity Index determined for each batch of thromboplastin reagents by manufactures.

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

  2. Obtain the mean normal prothrombin time (MNPT). 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.

  3. 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.

  4. Calculate the International Normalized Ratio using the numbers above as an example. 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.

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