Determination of the mean normal prothrombin time for assessment of international normalized ratios. Usefulness of lyophilized plasma.

To report a Prothrombin Time (PT) as International Normalized Ratio in controlling oral anticoagulant therapy, the Mean Normal PT (MNPT) is required. To correct for methodological differences in performing the PT test, each laboratory should determine its own MNPT for each batch of reagent using fresh blood samples from a large number of normal individuals. This would be a laborious procedure. Two models for simplified assessment of MNPT were investigated by two laboratories in a collaborative study. According to the models, the MNPT of a new batch of reagent is calculated, using the PT of a lyophilized control plasma measured with the new batch and a reference batch, as well as the MNPT of the reference batch obtained with fresh samples. Experimental results were obtained with 19 batches of bovine thromboplastin, 4 lyophilized normal control plasmas and fresh blood samples of 40 normal individuals. The PTs of the 4 lyophilized normal control plasmas were not identical to the MNPT of the fresh normal samples and also different from each other. Therefore, the uncorrected PTs of these control plasmas cannot be used as MNPT. In general, there was good agreement between measured and calculated MNPT, although some control plasmas gave better results than others. There were no significant differences between the results obtained by both calculation models.