The estimates of efficiencies for the direct smear and zinc sulfate technic and the prevalence of certain protozoan infections in an institutional population are presented and compared with the results of a similar study in the literature. In regard to E. histolytica the efficiency of the direct smear was estimated to be 27 per cent, the zinc sulfate technic 59 per cent and the combination of the two technics 62 per cent. The superiority of the zinc sulfate technic is evident.
The efficiencies indicated herein may be used as a guide in planning other surveys. However, those determined in the present study apply only to this particular survey and it is recommended that in other surveys the parasitologist determine in a similar manner the efficiencies for his study so that valid estimates of prevalence rates can be made.
It might be emphasized that while the absolute efficiencies of the various technics apply only to the study herein reported, the relative efficiencies of the technics are probably valid for any survey in which the protozoa are primarily in the cystic stage.
On the basis of the estimates of efficiency obtained, the recommendation of repeated zinc sulfate examinations on different stools, in contrast to use of various technics on the same stool, is made.
In a series of repeat zinc sulfate examinations performed on 28 persons known to harbor E. histolytica, 71 per cent of the infected persons were detected on the first examination, 82 per cent through the second, 86 per cent through the third, 96 per cent through the fourth and 100 per cent through the fifth examination.
A simplified method of performing the zinc sulfate technic is described and it is felt that this will be of value to the parasitologist for stool surveys conducted in the future.
St. John's Medical College, Shanghai, China.
The authors are indebted to Mr. Stanley B. Ward, Research Technician, National Institutes of Health and Mr. Louis Cook, Technician, Division of Laboratories, and Research, New York State Health Department, for technical assistance.