At a Glance
The presence of diarrhea, after the appropriate epidemiologic exposure, should make the clinician consider the possibility of an enteric protozoal infection.
There are, essentially, three types of tests available for the detection and identification of intestinal protozoal parasites: rapid immunoassay or immunofluorescent screening tests, the ova and parasite examination (O&P), and special stains to identify select parasites.
Screening Immunoassays and Immunofluorescent Assays
These are commercially-available assays for the detection of Giardia lamblia and Cryptosporidium species. These assays may be used on formalin-fixed stool specimens and have heightened sensitivity compared with the traditional O&P examination, since they detect antigen (i.e., intact organisms do not have to be present for identification). These are commonly used as a screening test in North America, since Giardia lamblia is the most commonly occurring intestinal protozoal parasite.
These assays also commonly contain a separate position that detects the antigen of Cryptosporidium species. This is useful, since Cryptosporidium is difficult to detect in the O&P examination and may be missed. False-positive reactions have occurred with these assays, so microscopic confirmation of positive results is recommended. There is also a commercially-available assay for Entamoeba histolytica, but this requires fresh, rather than formalin-fixed, stool, which has limited the usefulness of this assay.
The Ova and Parasite (O&P) Examination
The O&P examination is useful for detecting a wide variety of protozoal parasites, as well as the eggs of helminthes and the larvae of Strongyloides. Both the cyst and trophozoite forms are discernible in these preparations. There are two parts to this examination that follow specimen processing. The first is an iodine-stained wet preparation, which detects the cysts of protozoa, larvae of Strongyloides, and the eggs of helminthes. The second is the trichrome-stained preparation, which detects both the cysts and trophozoites of protozoa. The trichrome stained stool preparation is examined using oil-immersion microscopy, which affords the identification and differentiation of protozoa.
There are certain parasites not detected or detected poorly in the O&P preparation. If these are clinically-suspected, then select special stains should be ordered. Cryptosporidium species, Cyclosporia cayetanensis, and Cystoisospora(Isospora) belli are protozoa that stain with the modified acid fast stain. This or an immunofluroescent stain should be used to confirm a positive Cryptosporidium enzyme immunoassay result, since false positives have occasionally occurred with this test. Although Cyclospora and Cystoisospora (Isospora) may be detected in the O&P examination, this stain is easier to interpret and is confirmatory.
Finally, although the microsporidia have been officially recategorized as fungi, the modified trichrome stain used for their detection is often performed in the parasitology laboratory. A special request for this stain is needed for the modified trichrome stain for microsporidia, if an infection caused by these microorganisms is suspected.
Positive for any of the following organisms is indicative of an intestinal protozoal parasite:
There are several commensal protozoa, which may be present in the stool following exposure to contaminated food or water. These are commonly reported, although they are non-pathogenic:
Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?
There are no descriptions of medications that affect the laboratory tests for protozoal parasites.
What Lab Results Are Absolutely Confirmatory?
The morphologic detection of a protozoal parasite by an experienced parasitologist is confirmatory.
Positive enzyme immunoassay results should be confirmed with the O&P examination if Giardia or E. histolytica is detected, whereas the modified acid fast stain should be used to confirm the presence of Cryptosporidium.
No confirmatory tests are necessary for the morphologic identification of protozoal parasites. If the parasitology technologist is inexperienced, then an experienced technologist should confirm the findings. Enzyme immunoassays should be confirmed by a morphologic method, as described.
There are no known factors that affect the laboratory results.
There is a standard practice, which is usually not justified, of ordering three O&P examinations. The only time additional O&P evaluations are warranted is when the first O&P examination is negative and there is a high index of suspicion that the patient is infected by either a helminthic or a protozoal parasite. Similarly, if Giardia or Cryptosporidium is the suspected pathogen, then the Giardia/Cryptosporidium screening enzyme immunoassay should be used because of the superior sensitivity of this assay.
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- At a Glance
- Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?
- What Lab Results Are Absolutely Confirmatory?