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 2 0 healthy individuals is the recommended way to derive a normal value for the prothrombin ratio. This is generally believed to give a reason able representation of the normal in the local population, but there m ay be difficulty in obtaining a good representative sample in some cen tres. The alternative of a uniform lyophilized 'normal' plasma has bee n suggested. The comparative value of a widely used commercial 'normal ' plasma and the MNPT has been assessed in an international study at 3 7 centres. Two common thromboplastins were tested using the local coag ulometer methods. The variability of the results and their relative pe rformance in local thromboplastin calibration to derive the Internatio nal Sensitivity Index (ISI) were assessed. The reliability of the two types of normal in local system ISI calibration with different lyophil ized plasma calibrants has also been tested. When the lyophilized 'nor mal' was substituted for the MNPT in ISI determination no appreciable difference in the degree of correction for coagulometers effects on In ternational Normalized Ratios was found.