Source of analytic variation in ligand assay: a study based on data from the 1976 and 1981 Basic Ligand Assay Surveys of the College of American Pathologists.

Six paired specimens distributed to laboratories in 1981 at approximately three-month intervals and four paired specimens distributed to laboratories in 1976 at intervals of three to nine months for analysis by the College of American Pathologists Survey form the basis for this study. Twelve of 37 (35%) of pool-analyte-technic combinations yielded significantly changed mean values in 1976, while 34 of 196 (17%) pool-analyte-technic combinations yielded significantly changed values in 1981. Probable instability in thyroxine and folate was demonstrated in the 1981 control pools. Precision generally continued to improve from 1976 to 1981. The poorest precision now is observed in the analysis of peptide hormones. A majority of the observed analytic variation during 1981 for most analytes relates to extralaboratory factors. Improvement in performance is largely dependent on intermanufacturer standardization of procedures and long-term maintenance of equivalence of the results of kit procedures by each manufacturer.