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 concentrations, ethoxyresorufin O-de-ethylase (EROD) activity and protein concentrations were compared during a practical workshop. The purity of resorufin standards used by participants in their own laboratories (estimated from molar absorbance measurements) was highly variable and ranged from 18% to (apparently) > 100%. Estimates of the resorufin concentrations in two ‘unknown’ solutions analysed by participants approximately covered a 6-fold range, but when these estimates were corrected for the variation in the purity of the standards used, this range narrowed to (approximately) a 2-fold range. Estimates of protein concentration varied over a 2-fold range depending on the choice of method and standard; 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 the participants' choice of methods (and normalised 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 value. This was greatly reduced (SD became < 20% of the mean value) when all participants used a common method. If the purity of standard resorufin is controlled and if investigators agree on a set of standard conditions for the measurement of EROD, data from different laboratories could be compared with reasonable success.