Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1404 page)

   This test is ordered for diagnosis of common enteric parasitic pathogens in fecal specimens.

   Parasitic infections present with a remarkable diversity of signs and symptoms. Indications for testing for endemic parasitic infections may be fairly straightforward. Clinicians must have a high index of suspicion for parasitic infection when patients present with symptoms after travel to regions where other parasites are endemic.

   Use

   Specimens should be examined visually to identify any macroscopic parasitic forms, like pinworms or tapeworm proglottids. The routine O & P examination of stool includes three components: a direct wet mount (unpreserved liquid stool only), wet mount of stool concentrate (formalin-fixed specimen), and preparation of a permanent stained smear (polyvinyl alcohol [PVA]-fixed specimen).

   The direct wet mount may provide a rapid diagnosis and demonstrate motility of trophozoites in heavily infected patients.

   The concentrated stool wet mount, prepared from the formalin-fixed stool specimen by sedimentation or flotation, provides for detection of protozoal cyst forms, oocyst of coccidian parasites, microsporidia, and helminth eggs and larvae.

   The permanent smear, made from the PVA-preserved stool specimen, provides the best morphology for identification of parasites and recognition of artifacts as well as providing a permanent slide that can be referred for identification, if necessary. Permanent stains should be used to confirm the identification of any parasite detected by wet mount.

   
Turnaround time:
48–72 hours.

   Special Collection and Transport Instructions

   Stool should be submitted in clean containers with tight-fitting lids. It is not necessary to use sterile containers. Stool specimens collected by swab, from the toilet, or on toilet paper are not appropriate. The detection of parasites may be inhibited by intestinal contrast (barium sulfate), mineral oil, bismuth medications, antidiarrheals, and medications with antiparasitic action. Delay specimen collection for 1–2 weeks after the use of these agents.

   Submit stool during the diarrheal phase of disease. Trophozoite forms may be detected only in diarrheal stool; cyst forms are more common in formed stool.

   At least three stool specimens, collected on separate days, should be submitted within 10 days. A purgative agent, such as magnesium sulfate, improves the detection of intestinal parasites. Six specimens, collected on different days over a 2-week period, should detect more than 90% of amebic infections.