Fecal concentration has become a routine procedure as a part of the complete ova and parasite examination for parasites and allows the detection of small numbers of organisms that may be missed by using only a direct wet smear. There are two types of concentration procedures, sedimentation and flotation, both of which are designed to separate protozoan organisms and helminth eggs and larvae from fecal debris by centrifugation and/or differences in specific gravity.
Sedimentation methods (using centrifugation) lead to the recovery of all protozoa, oocysts, spores, eggs, and larvae present; however, the preparation contains more debris. If one technique is selected for routine use, the sedimentation procedure is recommended as being the easiest to perform and least subject to technical error.
A flotation procedure permits the separation of protozoan cysts, coccidian oocysts, microsporidian spores and certain helminth eggs and larvae through the use of a liquid with a high specific gravity. The parasitic elements are recovered in the surface film, and the debris remains in the bottom of the tube. This technique yields a cleaner preparation than does the sedimentation procedure; however, some helminth eggs (operculated eggs and/or very dense eggs such as unfertilized Ascaris eggs) do not concentrate well with the flotation method. The specific gravity may be increased, although this may produce more distortion in the eggs and protozoa. Laboratories that use only flotation procedures may fail to recover all of the parasites present; to ensure detection of all organisms in the sample, both the surface film and the sediment should be carefully examined. Directions for any flotation technique must be followed exactly to produce reliable results.
Formalin-Ethyl Acetate Sedimentation Concentration
By centrifugation, this concentration procedure leads to the recovery of all protozoa, eggs, and larvae present; however, the preparation contains more debris than is found with the flotation procedure. Ethyl acetate is used as an extractor of debris and fat from the feces and leaves the parasites at the bottom of the suspension. The formalin-ethyl acetate sedimentation concentration is recommended as being the easiest to perform, allows recovery of the broadest range of organisms, and is least subject to technical error.
The specimen must be fresh or formalinized stool (5 or 10% buffered or nonbuffered formalin or sodium acetate-acetic acid-formalin [SAF]). Polyvinyl alcohol (PVA)-preserved specimens can also be used.
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