Variation in weight of stool samples prepared by the Kato-Katz method and its implications

We investigated both the extent of the variation in weight of stool samples prepared by the Kato-Katz method and its influenced on egg counts and commonly used group parameters of infection derived from them. In a first study group of 795 people, the total mean weight of stool aliquots, prepared with templates designed to contain 28.3 mm3, was 23.0 mg with 95% of the individual values lying between 12.0 and 34.0 mg. Minimum and maximum values were 2.4 and 49.5 mg, respectively. Frequency distributions of the individual weights, in series of slides prepared by different laboratory assistants, showed significant differences. In a second study group of 199 people, duplicate series of slides were prepared and variations in the weight of examined stool were related to variations in egg count. The correlation between repeated individual sample weights in this series was poor, but the correlation between eggs counts was good. This was translated, at aggregate level, in very similar classifications in egg count categories. This classification was also hardly influenced by the choice of the conversion factor to transform eggs counts per slide into eggs per gram. At the individual level, the variability in egg counts far outweighed the variability in sample weight and was not clearly related to it. We therefore concluded that variations in weight of examined stool are considerable, but account for only a minimal part of the important egg count fluctuations generally observed.