Evaluation of the Kato-Katz thick smear and formol ether sedimentation techniques for quantitative diagnosis of Schistosoma mansoni infection

The present study compared the Kato-Katz thick smear and formol ether sedimentation techniques in the diagnosis of Schistosoma mansoni infections. A stool specimen was collected from 915 individuals representing a high prevalence community (63.3%) and from 471 individuals representing a relatively low prevalence village (40%). The overall sensitivity of a single Kato-Katz smear was 70.8%, and it increased with each additional slide to reach 91.7% on examining four smears. However, the sensitivity was 83.3% when using the formol ether sedimentation technique. In terms of quantitative analysis, the geometric mean egg count was 94 eggs per gram (epg) of stool by two Kato-Katz smears, and 43 epg by the sedimentation technique. This means that more than 50% of eggs were missed when using the sedimentation technique, a fact that should be taken into consideration when relating infection level with morbidity.