Special report: a simple system for the derivation of International Normalized Ratios for the reporting of prothrombin time results with North American thromboplastin reagents.

The World Health Organization international scale of reporting prothrombin time results is based on the calibration of thromboplastins against an international reference preparation to derive an International Sensitivity Index (ISI). Once the ISI has been assigned to an individual thromboplastin reagent, the derivation of International Normalised Ratios (INRs) for reporting results depends on mathematic formulae requiring a special calculator or mathematic tables. This causes difficulties and errors. A simplified system for interpretation of INRs with the range of thromboplastins widely used in North America is therefore presented, which obviates the need for mathematic procedures for the derivation of INR equivalents. It should thus facilitate the application of the INR system and of safer therapeutic ranges.