Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1405 page)

   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.