Re-evaluation of the formalin-ether sedimentation method for the improvement of parasite egg recovery efficiency

The formalin-ether sedimentation (FES) method is considered as reliable method of fecal examination for the detection of parasites. In this study, we re-evaluated several aspects of FES such as (i) pretreatment of feces; (ii) filtration of fecal suspensions; (iii) test-tube material and (iv) substitution of ether by other organic solvents as to see an improvement in parasite egg recovery. The egg count was represented by the number of ova detected per 100 μg of sediment. Pre-treatment of feces with formalin (pH 7) increased egg detection rate remarkably compared with original FES method. Use of three layers of gauze dramatically reduced the sediment in the final product, and led to an increase in the number of ova detected. Use of polypropylene test tubes instead of glass test tubes also increased the number of egg detection. None of the organic solvents used to replace the ether produced better results. Based on these findings, we proposed a modified FES procedure. Further, we also compared the parasite positive rate and the number of ova recovered by using original FES and the modified FES procedures by examining 112 fecal samples collected from school children of parasite endemic area in Nepal. Feces collected from Nepal had many parasite ova, and these fecal samples barely displayed false-negative results even by method with low sensitivity. When the mean number of Hemenolepis nana, hookworm, T. trichiura, and A. lumbricoides ova recovered by original FES and the modified FES methods was compared, the values obtained by modified FES were superior (higher). This result suggested that the modified FES is effective and better for the recovery of parasite ova in areas of low-intensity parasitic infection.