International multicenter international sensitivity index (ISI) calibration of a new human tissue factor thromboplastin reagent derived from cultured human cells

Summary.  The international sensitivity index (ISI) of the first working standard of Simplastin HTF, a new human tissue factor thromboplastin derived from cultured human cells, has been assessed in a calibration exercise in two Canadian and five European laboratories. Calibrations against international reference preparations (IRP) were performed for the manual method and six types of automated coagulometers that cover the majority of clotting endpoint principles in routine use. The ISI was method-dependent and varied between 1.03 and 1.29 when calibrated against rTF/95 (human IRP). The ISI was also dependent on the route of calibration. Compared with calibration against rTF/95, the ISIs obtained by calibration against RBT/90 (rabbit IRP) were on average 4.4% higher ( P  < 0.005). Considering the principle of 'like vs. like', the ISIs obtained by calibration against rTF/95 should be preferred.