The unreliability of the Kato-Katz technique limits its usefulness for evaluating S-mansoni infections

The Kato-Katz technique, a (semi) quantitative stool examination technique, is generally recommended for diagnosis and evaluation of Schistosoma manso ni infection by schistosome experts. However, egg counts are subject to imp ortant variability. In order to quantify the reproducibility of egg counts using the Kato-Katz technique, field data of 1255 observations on 299 subje cts infected with Schistosoma mansoni were analysed. Agreement between repe ated observations was assessed both categorically (kappa statistic) and con tinuously (analysis of variance). The day-to-day variation of egg counts wa s much greater than the variation due to different observers or different s lides. The quantitative reproducibility was low: the weighted kappa statist ic was 0.39 between specimens of different days, 0.62 between slides of the same specimen and 0.81 between observers of the same slide. Therefore the classification of individual patients into groups based on egg counts, used as a measure of morbidity, must be interpreted with great care, especially in longitudinal studies. Usefulness of the Kato-Katz technique appears lim ited. Its reproducibility is low. It cannot be recommended as a routine tes t in a primary health care setting or in a hospital laboratory because safe ty and detection of other parasites are better assured by other techniques. It can be used in epidemiological studies and evaluation of schistosomiasi s control programmes, but here too, other techniques might be preferred.