Interlaboratory comparison of cholinesterase assay measurements

Twelve wildlife toxicology laboratories participated in an interlaboratory survey of cholinesterase (ChE) assays to determine comparability of absolute ChE values and estimates of ChE inhibition from organophosphorus insecticide-dosed birds and to examine the type and consistency of methods employed by laboratories for the measurement of ChE Supernatants of brain from northern bobwhite quail (Colinus virginianus) were prepared from untreated (UT) and methyl parathion (MP)-treated birds (5 mg/kg). Assays also were conducted on a commercial preparation of acetylcholinesterase from an electric eel. All laboratories performed the ChE assays using the colorimetric assay with temperature and read times varying by laboratory. There was substantial variation in the results of ChE analyses among participating laboratories The ChE values from untreated birds ranged from 2 6 to 28.0 μmol acetylthiocholine iodide (ACTI) hydrolyzed/min/mg tissue (mean = 14.8 sd = 7 1). For the MP-treated group, ChE activity values showed a range of 1 5 to 15 3 μmol ACTI hydrolyzed/min/mg tissue (mean = 8 1, sd = 3 9) Percent inhibition values of MP-treated compared to untreated birds ranged from 37 to 51% for reported results and 37 to 49% for corrected data (activity calculated from reported absorbance data) Laboratories were most consistent in their estimates of percent inhibition of ChE activity Differences among laboratories were partially explained by differences in the methods employed for the analysis of ChE. Assay temperature and duration appeared to affect the absolute values the most However, differences in reported ChE activity values existed even when methods were very similar These results do not support the concept of using published normal ChE values from other laboratories to assess ChE inhibition in field-collected samples