Summary
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.
