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Detection and identification of parasitic protozoa and the eggs and larvae of parasitic helminths
A variety of different parasites may be found in stool specimens, duodenal aspirates, respiratory specimens, liver cyst aspirates/abscesses and tissues. These parasites may include protozoa (microscopic unicellular eukaryotes) and helminths (aka worms). Infection is often asymptomatic, but symptoms range from diarrhea and malnutrition, intestinal obstruction, and rarely, death.
The most common intestinal reported parasites in stool specimens are Giardia intestinalis (aka Giardia duodenalis, Giardia lamblia) and Cryptosporidium species. Both parasites may cause watery diarrhea and are endemic in the United States. The best tests for these 2 common parasites are parasite-specific fecal antigen tests (GIAR / Giardia Antigen, Feces and CRYPS / Cryptosporidium Antigen, Feces).
Other parasites are less commonly seen in the United States, and the stool parasitic exam is the appropriate test for their detection. See Parasitic Investigation of Stool Specimens Algorithm in Special Instructions for determining which test should be ordered based on the patientâ€™s exposure history and risk factors.
If positive, organism identified
A positive result indicates the presence of the parasite but does not necessarily indicate that it is the cause of any symptoms. Some strains of protozoa are nonpathogenic and some helminths cause little or no illness.
If stool specimens are suspected of containing tapeworm segments or other adult worms, the suspected worm should be placed in 70% alcohol and submitted for PARID / Parasite Identification.
For optimal results, the specific test should be ordered for detection of Giardia, Cryptosporidium, microsporidia, Cyclospora, or pinworm.
This test is not appropriate for the detection of Acanthamoeba, Naegleria, microfilariae, malaria, trypanosomes, Toxoplasma, or Trichomonas.
Parasitic examination of a minimum of 3 stool specimens is required for the detection of > or =90% of some protozoal infections.
Parasitic infections are uncommonly acquired in the hospital setting. This test is not usually useful in patients hospitalized for more than 3 days.
Garcia L: Diagnostic Medical Parasitology. Fifth edition. Washington, DC, ASM Press, 2007