Prothrombin time standardization by correction of the Pivka inhibitor

The standardization of prothrombin time (PT) assays needs two steps: (1) calibration of PT assays towards a reference assay or reference thromboplastin, (2) correction of PT assays according to the calibration. The present recommended calibration by clotting times is favored for the linearity between assays; the clotting times of abnormal plasma are partly prolonged due to the protein induced by vitamin K absence (Pivka) inhibitor. Calibration by coagulation activities also demonstrated linearity between PT assays; the regression line for abnormal plasma deviated from the line of identity due to differences in sensitivity of assays for the Pivka inhibitor. The corrected PT assays demonstrated similar results using calibration by clotting time or coagulation activities, but the correction was simpler for coagulation activities. Patient plasma was the preferable material in calibration by clotting times as well as by coagulation activities. The corrections between the reference assay and other PT assays were equal to the differences in sensitivities for the Pivka inhibitor. The corrected PT assays did not differ from the reference assay by statistical analysis; any of the six PT assays tested might be used as reference assay.