The reliability of the mean normal prothrombin time of fresh plasmas and of the normal value from a lyophilized 'normal' plasma in prothrombin ratio determination.

The mean normal prothrombin time (MNPT) based on the fresh plasma of 20 healthy individuals is the recommended way to derive a normal value for the prothrombin ratio. This is generally believed to give a reasonable representation of the normal in the local population, but there may be difficulty in obtaining a good representative sample in some centres. The alternative of a uniform lyophilized 'normal' plasma has been suggested. The comparative value of a widely used commercial 'normal' plasma and the MNPT has been assessed in an international study at 37 centres. Two common thromboplastins were tested using the local coagulometer methods. The variability of the results and their relative performance in local thromboplastin calibration to derive the International Sensitivity Index (ISI) were assessed. The reliability of the two types of normal in local system ISI calibration with different lyophilized plasma calibrants has also been tested. When the lyophilized 'normal' was substituted for the MNPT in ISI determination no appreciable difference in the degree of correction for coagulometers effects on International Normalized Ratios was found.