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Statistical Evaluation of the Formalin-Ether (406th MGL) Fecal Sedimentation Concentration Procedure¹

Estimations of efficiency of the formalin-ether sedimentation concentration procedure for the detection of ten common intestinal parasites have been determined from an analysis of data obtained in multiple-stool examination series. Approximately 1,000 residents of four, rural, Japanese villages were examined in two five-stool series and findings were analyzed by the maximum-likelihood method of Sawitz and Karpinos. Mean estimates of efficiency for the detection of protozoan cysts were comparable to those computed by other investigators for the ZnSO₄ centrifugation-flotation technic. These estimates, in per cent, for the formalin-ether procedure were 60.2 for Entamoeba histolytica; 75.1 for E. coli; 60.4 for Endolimax nana; 55.7 for Iodamoeba bütschlii; and 44.5 for Giardia lamblia. Corresponding figures for helminths were: 91.9 for Ascaris lumbricoides; 92.2 for Trichuris trichiura; 78.9 for hookworm; 41.1 for Trichostrongylus sp.; and 47.6 for Schistosoma japonicum.

The efficiency of the formalin-ether procedure for the detection of some organisms, especially protozoan cysts, was observed to vary and, in some instances, the differences were probably significant. These variations are assumed to be due to a combination of factors including technical error, limitations of the analytical method and the examination procedure itself, possible biologic characteristics of the organisms that might affect their detection, and normal fluctuations in the mean infection intensity in the different population groups.

Re-examination of about one-half of the people within 60 days after the initial examination series permitted a better estimate of the reliability of the technic. The number of additional infections detected in the second examination series was about equal in all instances to the number of cases present in the first series, but absent in the second. The rate of misses for E. histolytica was no greater than the rate for more easily recognized forms. On the basis of the latter data, it was estimated that examination of five separate stools by the formalin-ether technic will detect about 97 per cent of amebiasis cases in a population. Similar data were also obtained for the other nine parasites.

¹ The authors wish to acknowledge the valuable contributions made in this study by Dr. S. S. Lin, Mr. S. Asakura, Mr. Y. Hishinuma and others who participated in the study.

² Address Dept. of Medical Zoology, Walter Reed Army Institute of Research, WRAMC, Washington 12, D. C.

³ Address Parasitology Branch, Sixth US Army Area Medical Laboratory, Fort Baker, California.

⁴ Tropical Research Medical Laboratory, APO 851, New York, New York.