EXAMINATION OF INTERNATIONAL NORMALIZED RATIO (INR) IMPRECISION BY COMPARISON OF EXACT AND APPROXIMATE FORMULAS

Aim-To evaluate the accuracy of the approximate linear formula of Inte rnational Normalised Ratio (INR) imprecision by formal mathematical an alysis. Methods-Using probability theory, an exact formula for the coe fficient of variation (CV) of the INR was derived. The CV from the app roximate formula was compared with the CV from the exact formula for I NR determinations between 1.0 and 10.0 with International Sensitivity Indices (ISIs) between 1.0 and 3.0 and prothrombin time ratio CVs betw een 1.0 and 10.0%. Results-When the ISI equals 1.0, the approximate fo rmula and the exact formula are equal. When the ISI is more than 1.0, the approximate formula overestimates the exact CV, but by less than o ne hundredth of the exact CV in the parameter ranges studied. The appr oximate formula is most accurate when laboratories achieve excellent p rothrombin time measurement precision and use sensitive thromboplastin s. Conclusions-The approximate formula provides a simple means for est imating the imprecision of the INR and is sufficiently accurate to war rant its use in clinical laboratories.