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.
Stool specimens should be transported to the laboratory as quickly as possible. Stool must be examined within 1 hour of passage (30 minutes for liquid or semiliquid stool) if direct wet mount is needed for detection of motile forms. If the transport to the laboratory will be delayed, the stool should be placed into preservative. Stool collection kits generally contain a vial of 10% formalin and a vial of PVA solution. The PVA vial is inoculated to give a 3:1 ratio of fixative to stool. The ratio for formalin should be 3:1 or greater. The stool must mixed be thoroughly with the preservative to ensure that parasitic elements do not degrade with storage. The formalin suspension is used to prepare a direct wet mount from concentrated material. The PVA-fixed material is used to prepare smears for permanent stains.
Three O & P examinations should be performed after therapy: 3 or 4 weeks after treatment for protozoal infection and 5 or 6 weeks after treatment for Taenia infection.
Special techniques are required for collection of duodenal specimens or specimens collected by endoscopy or other invasive techniques.
Interpretation
Expected results: Negative.
Positive results: Positive O & P examinations are associated with a high probability of parasitic infection or colonization. Identification of nonpathogenic parasites suggests exposure to unsanitary conditions; repeat testing should be considered in symptomatic patients.
Negative: A single negative test does not effectively rule out enteric parasitic infection. Sensitive detection of the infecting parasite may require additional testing of alternative techniques, like duodenal aspiration.
Limitations
For some enteric parasitic infections, specimens other than stool, like duodenal contents, may be required for diagnosis. Special techniques, like egg-hatching techniques, may be needed for sensitive detection of certain parasites.
Common pitfalls:
The submission of too few specimens limits the performance of stool O & P examination.
Special staining techniques are required for effective detection of certain enteric parasitic pathogens, like use of a modified acid-fast stain for detection of Cryptosporidium parvum or microsporidia.