Successful laboratory diagnosis of fungal infection depends in major part on collecting appropriate clinical specimens for investigation. These will differ depending on the suspected mycosis, site of infection, and clinical signs and symptoms.
Microscopic Examination of Clinical Specimens
Direct examination of clinical material is useful in diagnosing superficial and subcutaneous infections; histologic specimens can detect more deep-seated infections. Successful diagnosis depends on how distinctive the organism appears. It is a valuable procedure, as a tentative diagnosis can be made rapidly before growth occurs in culture. For example, in the diagnosis of cryptococcal meningitis, encapsulated budding yeast visualized with an India ink preparation can be detected in the CSF. Diagnosis of oropharyngeal candidiasis can be confirmed by Gram’s stain or by 10% KOH preparation of oral scrapings that will show yeasts, hyphae, and pseudohyphae. Histoplasma capsulatum can sometimes be detected in blood or bone-marrow smears prepared by Giemsa stain.
Culture provides a definitive diagnosis of a fungal infection but has several limitations. Isolation and identification can take several weeks, and results may be too late to guide treatment. Traditional blood-culture methods are relatively insensitive. Technical modifications have aimed at improving sensitivity and/or reducing the time to positive blood culture. These include biphasic media , lysis centrifugation , marketed as the DuPont isolator® system (DuPont, Wilmington, DE), and automated monitoring using the BacT/Alert® system (Organon Teknika Corporation, Durham, NC) or the BACTEC® system (Becton Dickinson Diagnostic Instrument Systems, Sparks, MD) . The sensitivity of these 2 automated systems is comparable to lysis centrifugation . Lysis centrifugation is time-consuming, and many microbiology labs find its use difficult to justify on a financial basis.
The immunogenic properties of fungal organisms have been exploited in tests for the presence of both fungal antigen and antibody in biologic fluids. The diagnostic value of tests for antigen or antibody reactions is highly dependent on the organism. Numerous methodologies have been used or are being developed with varying degrees of sensitivity And specificity. These include counter-immunoelectrophoresis, radioimmunoassay, latex agglutination, immunoblot, and enzyme-linked immunoassay (ELISA). Reliable tests are available for accurately diagnosing endemic infections such as coccidioidomycosis and histoplasmosis; similar successes have not been achieved with invasive aspergillosis and candidiasis. Biochemical assays for circulating metabolic products such as mannose and D-arabinitol using gas-liquid chromatography have been described, although none are in regular use in the clinical laboratory. DNA fingerprinting and polymerize chain reaction (PCR) techniques are areas of expanding development .
Radiographic imaging can be useful in certain circumstances, particularly in detecting early Aspergillus infection. Chest radiographs provide suggestive clues for respiratory mould infections, but high-resolution computerized tomography (CT) greatly enhances the diagnostic potential and can be useful to localize lesions for biopsy and to follow treatment.
Course Number: V035B.043001
This CME Expires on July 1, 2003; no tests will be accepted after this date.
This course is accredited by
The University of Pittsburgh School of Medicine, Center for Continuing Education