Influence of plasma volumetric errors on the prothrombin time ratio and International Sensitivity Index.

The International Sensitivity Index (ISI) for prothrombin time systems depends on the thromboplastin manufacturer's recommended method for use. The purpose of the present study was to investigate the influence of small deviations from the recommended sample volume on the prothrombin time ratio and ISI. Four commercial reagents were studied; three with low ISI and one with high ISI. The effects of volumetric errors on the ISI were used to assess the associated effects on the International Normalized Ratio (INR). The effect of 10% volume error on the INR was not greater than 5%. The effects with the three low-ISI reagents were slightly greater than those with the high-ISI reagent. It is recommended that each laboratory should check the volumes of sample and reagent used for the prothrombin time test.