Which stool tests are performed in the workup of cryptosporidiosis?

Updated: Nov 11, 2019
  • Author: Melinda B Tanabe, MD; Chief Editor: Michael Stuart Bronze, MD  more...
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Answer

Since the oocyst is small (4-6 µm), it requires staining to be identified via light microscopy. [44] Modified acid-fast staining procedure is useful for the identification of oocysts (which may be difficult to detect with routine stains, such as trichrome). Cryptosporidium species stain a pinkish-red color on a uniformly green background. Unlike the modified Ziehl-Neelsen acid-fast stain, this stain does not require the heating of reagents for staining (See the images below.). Other studies have found the fluorogenic stain auramine-phenol to be a more sensitive and faster option, which has been adopted by many laboratories as the standard staining method. [44, 46]

Cryptosporidium parvum oocysts revealed with modif Cryptosporidium parvum oocysts revealed with modified acid-fast stain. Against a blue-green background, the oocysts stand out with a bright red stain. Image courtesy of CDC DPDx parasite image library
Cryptosporidium oocysts revealed with modified aci Cryptosporidium oocysts revealed with modified acid-fast stain

The criterion standard for stool examination is the immunofluorescence assay, which is based on oocyst cell wall antigens targeted by specific fluorescent monoclonal antibodies. The main disadvantages for this method are the inability to process large amounts of samples and the need for a specialized microscope and technician. Antigen detection via enzyme-linked immunosorbent assays (ELISA) or enzyme immunoassay (EIA) has also been developed with variable sensitivities and specificities depending on the commercial kit used, limiting its use in epidemiological studies. Chalmers et al demonstrated that enzyme-linked immunosorbent assays (ELISA) and immunofluorescent tests (IFA) have sensitivities above 90%, significantly higher than that of modified Ziehl-Neelsen stains (75%). [46, 47]

In the past few years, various PCR-based commercial multiplex molecular assays to detect Cryptosporidium species have been approved by the FDA. These tests can detect various pathogens (eg, parasites, bacteria, viruses) that cause diarrhea. They are costly and highly sensitive and require careful clinical correlation so are not readily available in all laboratories. [44, 45, 48]


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