Interlaboratory comparisons of red blood cell cholinesterase activity

Comparisons of red blood cell cholinesterase values (AChE, EC 3.1.1.7) from different laboratories can be difficult. This study evaluated different methods of interlaboratory comparison. Eleven subjects gave triplicate samples for analysis by three commercial laboratories. Comparisons were made relative to the mid-point of each laboratory’s reference range (mid-range), and by transformation to Ellman assay activity in U/mL using published/derived conversion factors. To observe each laboratory’s reliability, four subjects submitted two additional duplicate samples to each laboratory. The mean intraspecimen variabilities were 3%, range 1%–8%; 4%, range 0–9%; and 13%, range 2%–24% for laboratories 1–3, respectively. The mean mid-range percent for laboratory 3 was substantially lower than the means for laboratories 1 and 2. Therefore, laboratory 3 values were adjusted based on the means for laboratories 1 and 2 and the mid-point suggested by laboratory 3’s assay kit. The mean AChE relative to these mid-points were 109%, 107%, and 107% for laboratories 1–3, respectively. Ellman activities were 19.0, 19.9 and 20.8 U/mL for laboratories 1–3, respectively. Intrasubject differences for both comparison methods showed agreement that approximated intraspecimen variabilities for each laboratory. Comparisons involving laboratory 3 showed the poorest agreement consistent with laboratory 3’s greater intraspecimen variation. AChE from different laboratories can be compared by both methods evaluated in this study. All comparisons are limited by the precision and reliability of the laboratories involved. Copyright © 1999 John Wiley & Sons, Ltd.