Use of Lyophilized Calibrant Plasmas for Simplified International Normalized Ratio Determination with a Human Tissue Factor Thromboplastin Reagent Derived from Cultured

Background: For monitoring of treatment with oral anticoagulants, the clotting time obtained in the pro-thrombin time (PT) test is transformed to the Interna-tional Normalized Ratio (INR) with use of a system-specific International Sensitivity Index (ISI). The calibrant plasma procedure (CPP) is an alternative ap-proach to INR calculation based on the use of a set of lyophilized plasmas with assigned INRs. Methods: With the CPP, a linear relationship is estab-lished between log(PT) and log(INR), using orthogonal regression. CPP was validated for Simplastin HTF, a new human tissue factor reagent derived from cultured human cells. CPP precision was assessed as the CV of the slope of the regression line. The accuracy of the CPP was determined by comparing the INR obtained wit