[International normalized ratio in the prothrombin test: clinical significance and use]

The safety and efficiency of therapy by peroral anticoagulants (PA) depend on a laboratory monitoring based on the prothrombin test (PT). The test is distinguished through its variability conditioned by different means of results' presentation as well as through the sensitivity of thromboplastin and a type of a device used in coagulation detection. WHO recommended, 1983, to standardize the thromboplastin preparations through adjusting their sensitivity (the so-called International Sensitivity Index--ISI) to blood coagulation defects induced by PA versus the primary international reference thromboplastin. Thromboplastin ISI as well as the mean normal prothrombin time (MNPT) of blood plasma are used to calculate the international normalized ratio (INR). The presentation of PT results as INR is justified exclusively for the PA-therapy stabilized patients. The INR system makes it possible to optimize the PA therapy only if the laboratory expert and clinician can clearly understand the PT standardization essence and observe the key WHO recommendations, i.e. definition of a coagulometer-specific ISI by manufacturing companies, estimation of MNPT by laboratories and use of the correct anticoagulant concentration.