Coccidioidomycosis laboratory tests
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: ; Vidit Bhargava, M.B.B.S [2]
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Overview
The fungal infection can be demonstrated by microscopic detection of diagnostic cells in body fluids, exudates, sputum and biopsy-tissue. With specific nucleotide primers C.immitis DNA can be amplified by PCR. It can also be detected in culture by morphological identification or by using molecular probes that hybridize with C.immitis RNA.
Staining & Culture
Usual KOH staining doesn't help in diagnosis of this fungal infection but organisms can be stained by using special stains such as papanicolaou stain or gomori methamine silver stain on sputum or other respiratory fluids. Even biopsy specimens can be stained by methamine silver stain to demonstrate spherules surrounded by inflammation. Coccidioides is easy to culture, grows in about 3-7 days at 37°C on simple media such as blood agar, however precautions needs to be exercised as it poses a significant health hazard to lab personnel.
Serology
These form the mainstay of commercial testing for the diagnosis of coccidioidomycosis. Several techniques are available such as the older Tube-precipitin method as well as the newer CF(complement fixation) and EIA(enzyme immuno assay) assays. EIA assay picks up IgM and IgG antibodies against the coccidioidal antigens. IgM antibodies are found early in the disease progression and are not useful for assessing prognosis, while IgG antibodies appear late and persist longer. CF titres can be used as a measure of disease progression and to diagnose meningitits by measuring CF antibodies in CSF. EIA based rapid diagnostic commercial kits are available but are not as sensitive or specific as lab based tests and have a risk of false positives.
PCR based methods
PCR assays are useful to detect a target gene one DNA has been extracted from the target tissue. It is highly sensitive and much safer compared to lab cultures. The C.immitans DNA can be amplified by PCR and then hybridised to molecular probes to aid in identification.