Formalin-fixed Stool Improves Performance of the Kato‑Katz method

Abstract Background Parasitic infections are a public health problem worldwide including in Thailand. An epidemiological survey for helminthiasis based on stool examination uses the Kato-Katz method as recommended by the WHO. Limitations of this method include the need for fresh stool as well as time requirements and a lack of quality control. These remain a big problem in parasite control policy. Our goal here was to use formalin-fixed stool samples to improve the diagnostic performance of the Kato-Katz method.Methods Stools were divided into two groups: formalin-fixed and unfixed stool groups at various time points and processes for the Kato-Katz method. Fresh Echinostome eggs were added to each stool group. The clearing process was increased by incubation with glycerol. Each group was observed and photographed under a light microscope. Parasite eggs were imaged and compared with the standard Kato-Katz method.Results The quality of Echinostome egg visualization from the slide of formalin-fixed stool was significantly better than the unfixed stool (p < 0.01). Stool fixed for seven days retains its normal Echinostome egg morphology. Incubation with glycerol for 1 h could increase the Kato-Katz performance by digesting the stool content and enhancing egg observation. Moreover, the result of Kato-Katz method using natural helminths infection fixed and fixed stool plus glycerol found the good quality of Opisthorchis viverrini and Taenia egg visualization, normal morphology with clear background of slide.Conclusions Formalin-fixed stool could be suitable and appropriate for the Kato-Katz method rather than fresh stool; it could be used for epidemiological studies.