Thromboplastin standardisation: calibrated lyophilised plasmas and coagulometer-specific international sensitivity...

The introduction of international reference preparations (IRP) of thromboplastins was an important step forward in the standardisation of laboratory monitoring of coumarin therapy for the treatment and prevention of thrombosis. In more recent years, the standard manual technique for prothrombin time (PT), which was used to monitor coumarin therapy, has been replaced gradually by a variety of different automated techniques in the laboratory. The international sensitivity index (ISI), assigned to a working thromboplastin by comparison to the IRP, is invalidated by the use of automated techniques, and is not necessarily consistent among analysers of the same type. Consequently, current recommendations are that individual laboratories should standardise their own thromboplastin with their routine method, using calibrated lyophilised plasmas which have international normalised ratios (INR) assigned to them.