Quality control in the diagnosis of Trichuris trichiura and Ascaris lumbricoides using the Kato-Katz technique: experience from three randomised controlled trials

Table 1 Two guidelines how to judge differences in faecal egg counts between the initial reading and the quality control reading of Kato-Katz thick smears

Guideline from the World Health Organization (WHO)	Internal guideline developed at the Swiss Tropical and Public Health Institute
“If the expert identifies a difference in the egg count per gram of stool^b of more than 10% and more than four eggs, he or she should re-read the slide with the microscopist and discuss the reasons for the discrepancy”. ^a [18]	Results are considered as inconsistent if there is a difference in presence/absence of a specific helminth species, or if differences in egg counts exceed (i) 10 eggs for Kato-Katz thick smears with ≤100 eggs, or (ii) exceed 20% for Kato-Katz thick smears with more than 100 eggs.

^aHow to handle differences in presence/absence of helminth eggs is not explicitly stated in the WHO guideline. Therefore we assume that differences in presence/absence of helminth eggs do not require re-reading as long as the difference does not exceed 4 eggs.
^bTo calculate eggs per gram of stool, the egg counts from a single Kato-Katz thick smear are multiplied by a factor of 24. When multiplying egg counts by a factor of 24, differences in egg counts of less or equally to four eggs are not possible. Therefore we assume for this current work that the WHO aimed to apply their guideline for egg counts for Kato-Katz thick smears rather than per gram of stool (as it is indicated within a footnote of the WHO document).

ISSN: 1756-3305