Diagnostic Techniques: The O&P Exam

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The page below is a sample from the LabCE course Intestinal Worms. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Diagnostic Techniques: The O&P Exam

The most common method of diagnosing helminth parasites mentioned in this course is microscopically detecting eggs in a stool specimen. Three stool specimens are usually recommended because the shedding of eggs can be inconsistent. Although permanent stained smears are often routinely part of Ova and Parasite (O&P) exams, they are not quite as useful for worms as for diagnosing the intestinal protozoa. Direct wet preparations can be made immediately, but concentration techniques such as the formalin-ethyl acetate procedure are often necessary to find them. Since helminth eggs are typically larger than most protozoa, they often can be found by using 100x or 400x (Low or High dry) magnification, especially when examining the concentrate.
Because concentration techniques can be useful for most helminth eggs, it is recommended that the stool be preserved in 10% formalin rather than the PVA (polyvinyl alcohol) fixative. If the stool arrives in the PVA fixative, the only procedure that can be performed is making a permanent stained smear. Although the eggs can be seen on stained smears, if their numbers are low, they could be missed.
1. DPDx. "Stool Specimens - Specimen Processing." CDC.gov, 3 May 2016, https://www.cdc.gov/dpdx/diagnosticprocedures/stool/specimenproc.html

The typical processing recommended by the CDC (1).