The effect of sample size on fresh plasma thromboplastin ISI determination

The possibility of reduction of numbers of fresh coumarin and normal plasma s has been studied in a multicentre manual prothrombin (PT) calibration of high international sensitivity index (ISI) rabbit and low ISI human referen ce thromboplastins at 14 laboratories, The number of calibrant plasmas was reduced progressively by a computer program which generated random numbers to provide 1000 different selections for each reduced sample at each partic ipant laboratory. Results were compared with those of the full set of 20 no rmal and 60 coumarin plasma calibrations. With the human reagent, 20 coumar ins and seven normals still achieved the W.H.O. precision limit (3% CV of t he slope), but with the rabbit reagent reduction < 50 coumarins with 17 nor mal plasmas led to unacceptable CV, Little reduction of numbers from the fu ll set of 80 fresh plasmas appears advisable. For maximum confidence, when calibrating the lSI of a new reagent, it would always seem worthwhile to us e the full number of patients and normal plasmas at each centre for the ini tial calibration, i.e. 60 fresh coumarin and 20 fresh normal plasmas. Small er numbers should achieve the required precision with the lower ISI reagent .