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- A. To provide calibration of local prothrombin time systems to accord with the WHO prothrombin time (PT) standardisation scheme ("the existing device"). - B. To avoid the need for local availability of an international reference preparation (IRP) for thromboplastin. IRP are not normally available except to national control laboratories and manufacturers. . . - C. To avoid the need for the performance of the manual PT- technique required for conventional WHO calibrations. Most centres have now discarded the manual technique for automated procedures and are no longer skilled in the manual PT technique. - D. To avoid the requirement for fresh plasma donations from 60 patients stabilised on warfarin treatment required for the conventional WHO calibrations. - E. To facilitate serial calibration studies to check against drift of automated instruments or the effect of batch changes of commercial thromboplastin reagent. - F. To provide a rapid result. A PT system calibration with the Calibrants takes about half an hour whereas a conventional WHO calibration requires several days. Five days are recommended by the revised 1999 WHO protocol.

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