AN INTER-LABORATORY COMPARISON OF MEASUREMENTS OF ETHOXYRESORUFIN O-DE-ETHYLASE ACTIVITY IN DAB (LIMANDA-LIMANDA) LIVER

Methods used in ten laboratories for estimating resorufin concentratio ns, ethoxyresorufin O-de-ethylase (EROD) activity and protein concentr ations were compared during a practical workshop. The purity of resoru fin standards used by participants in their own laboratories (estimate d from molar absorbance measurements) was highly variable and ranged f rom 18% to (apparently) > 100%. Estimates of the resorufin concentrati ons in two 'unknown' solutions analysed by participants approximately covered a 6-fold range, but when these estimates were corrected for th e variation in the purity of the standards used this range narrowed to (approximately) a 2-fold range. Estimates of protein concentration va ried over a 2-fold range depending on the choice of method and standar d; this variation was not reduced when all participants used a common method, although this may reflect lack of familiarity with the method chosen. EROD activity measured under different conditions reflecting t he participants' choice of methods (and normalized to sample volume to eliminate the effects of variance in protein determinations) was very variable: the SD of the measurements was almost equal to the mean val ue. This was greatly reduced (SD became < 20% of the mean value) when all participants used a common method. If the purity of standard resor ufin is controlled and if investigators agree on a set of standard con ditions for the measurement of EROD, data from different laboratories could be compared with reasonable success.