Effect of variable and low analytical recovery on sensitivity of Kato-Katz, Lutz and Ritchie methods for diagnosis of schistosomiasis

While new immunological or molecular diagnostic methods for schistosomiasis are currently under development and evaluation, egg-detection methods remain the most widespread available tool despite their limitations to diagnose light infections. Thick fecal smear, or the Kato-Katz method (KK), is recommended for populational screening. Seeding experiments were conducted to evaluate and compare sensitivity of KK, spontaneous sedimentation (Lutz or Hoffmann, Pons & Janer) and ether/ethyl-acetate centrifugation (Ritchie) methods for diagnosis of individual patients. Groups of 20, 40, 60, 80 and 100 S. mansoni eggs were introduced into 1 g aliquots of normal feces, repeated 5–8 times. Samples were totally screened at the microscope for enumeration of recovered eggs, though only the first two slides were considered in order to represent recommended clinical practice. Owing to a combination of losses during sample processing and partial sample analysis, recovery was very low for all methods. Modelling to evaluate analytical recovery and its implications for detection demonstrated a lower detection limit of the Ritchie method. The present data support recommendations in favor of the Ritchie method as a diagnostic tool for individual patients in comparison with KK and spontaneous sedimentation.