THE IMPORTANCE OF LIKE TO LIKE ISI CALIBRATIONS WITH FREEZE-DRIED PLASMAS

Aim-To assess reliability of like to like and cross species calibratio ns using two types of certified freeze dried plasma calibrants-artific ially depleted of vitamin K clotting factors, and from coumarin treate d patients. Methods-Six ECAA national control laboratories provided ce rtified values for the freeze dried plasmas in terms of the human plai n international reference preparation (IRP) (BCT/441) with the manual prothrombin time technique. Eight other ECAA national laboratories det ermined international sensitivity index (ISI) values in full fresh pla sma same species and cross species WHO calibrations against a low ISI human IRP (BCT/441) of the ECAA low ISI human thromboplastin and high ISI ECAA rabbit thromboplastin. Parallel calibrations were performed u sing the certified values. Results-Calibrations on fresh plasmas of th e human ECAA reference thromboplastin (stated ISI = 0.95) gave ISI of 0.957 against the human IRP and 1.66 against the rabbit IRP. The ECAA rabbit (stated ISI = 1.67) gave an identical value on the fresh plasma calibration v the human IRP. With freeze dried depleted plasmas certi fied in terms of the human IRP, the ISI of the ECAA human was 1.01, bu t the ECAA rabbit (stated ISI = 1.67) gave a low ISI of 1.47. The free ze dried coumarin plasmas gave an ISI of 0.943 for the ECAA human but only 1.493 for the ECAA rabbit. Conclusions-Fresh plasmas give reliabl e ISI when calibrating thromboplastins in same species and cross speci es calibrations. Freeze dried plasmas certified in terms of a single I RP, whether artificially depleted or of coumarin plasma origin, cannot be used for calibration of dissimilar thromboplastins.