Estimating sensitivity of the Kato-Katz technique for the diagnosis of Schistosoma mansoni and hookworm in relation to infection intensity

Abstract

The Kato-Katz technique is the most widely used diagnostic method in epidemiologic surveys and drug efficacy trials pertaining to intestinal schistosomiasis and soil-transmitted helminthiasis. However, the sensitivity of the technique is low, particularly for the detection of light-intensity helminth infections. Examination of multiple stool samples reduces the diagnostic error; yet, most studies rely on a single Kato-Katz thick smear, thus underestimating infection prevalence. We present a model which estimates the sensitivity of the Kato-Katz technique in Schistosoma mansoni and hookworm, as a function of infection intensity for repeated stool sampling and provide estimates of the age-dependent 'true' prevalence. We find that the sensitivity for S. mansoni diagnosis is dominated by missed light infections, which have a low probability to be diagnosed correctly even through repeated sampling. The overall sensitivity strongly depends on the mean infection intensity. In particular at an intensity of 100 eggs per gram of stool (EPG), we estimate a sensitivity of 50% and 80% for one and two samples, respectively. At an infection intensity of 300 EPG, we estimate a sensitivity of 62% for one sample and 90% for two samples. The sensitivity for hookworm diagnosis is dominated by day-to-day variation with typical values for one, two, three, and four samples equal to 50%, 75%, 85%, and 95%, respectively, while it is only weakly dependent on the mean infection intensity in the population. We recommend taking at least two samples and estimate the 'true' prevalence of S. mansoni considering the dependence of the sensitivity on the mean infection intensity and the 'true' hookworm prevalence by taking into account the sensitivity given in the current study.

Fig 1. Prevalence of hookworm infection in relation to age for the three studies in Côte d’Ivoire, including 95% BCI indicated by shaded areas, and observed prevalence as dashed line.

Fig 2. Prevalence of S. mansoni infection in relation to age for three studies in Côte d’Ivoire, including 95% BCI indicated by shaded areas, and observed prevalence as dashed line.

Fig 3. Relation between S. mansoni and hookworm infection intensity with Kato-Katz diagnostic sensitivity, including 95% BCI for k equal to 1, 2, and 3 samples per individual.

Fig 4. Estimated relation between sensitivity and arithmetic mean S. mansoni infection in a population with α = 0.07 and r = 1.0 for one to four samples.