QUALITY-CONTROL OF KATO SLIDE COUNTS FOR SCHISTOSOMA-MANSONI - A REVIEW OF 12 YEARS EXPERIENCE IN KENYA

A total of 19 annual or biannual audits were performed over a 12-year period by an independent microscopist on randomized subsamples of Kato slides examined for Schistosoma mansoni eggs by Kenyan microscopists from the Division of Vector-borne Diseases (DVBD). Tile recounts were invariably lower than the originals owing to some deterioration of the preparations between counts, but the two were strongly correlated: si gnificant regressions of recounts on counts taking up 80-90% of the ob served variance. Observer bias differed significantly between microsco pists but remained stable over time, whereas repeatability of recounts on counts dropped slightly in periods of maximum work load but did no t vary systematically with time. Approximately 7% of the counts and re counts disagreed on the presence or absence of eggs, but less than a t hird of these were negatives that were found positive on recount. Fals e negatives dropped to 1.3% if duplicate counts were considered. The p erformance of the Kenyan microscopists was remarkably high and consist ent throughout the 12-year period. This form of quality control is sui table for projects where limited funds preclude full-time supervisors using more sophisticated systems.