Standardization of clinical cholinesterase measurements

Previous studies showed that commonly used kits for measuring cholinesterases were not optimal for determining acetylcholinesterase (AChE) activity. Clinical use of different kits and methodologies resulted in AChE levels being reported in different units and activities that were not reproducible among laboratories. Findings such as these led to a revision in California regulations (covering AChE measurements for pesticide worker safety) calling for clinical laboratories to standardize their findings. The laboratories were contacted and invited to participate in a split-sample study of human blood AChE and nonspecific cholinesterase (BChE) assays. Participating laboratories measured erythrocyte (RBC) AChE and/or plasma BChE from undiluted and 50% diluted blood, according to their practices. Aliquots of blood samples were shipped to University of California Davis for measurement, using an optimized semiautomated plate reader version of the method of Ellman. Nine of 25 laboratories sent samples for comparison. Two others performed their own comparisons and submitted data to the state. Best correlations were obtained with BChE activity. Correlations (r(2)) were .88 or above for four of five laboratories for BChE, and above .9 for two of seven laboratories for AChE. Reasons for poor correlations may include difficulties in pipetting RBCs, storage, and processing. A bovine AChE RBC ghost "standard" was devised and tested. Activity of the preparation was maintained at -70 degrees C for approximately 11 months. A test with an East coast laboratory resulted in a high correlation, demonstrating the reliability of the RBC ghost standard and that one laboratory can replicate the AChE findings of another. The overall poor correlation of interlaboratory cholinesterase results points to the need to further standardize sample handling and assay methods.