Fungal Diagnostics

Abstract

The field of fungal diagnostics encompasses tests that are performed to help diagnose fungal disease, guide its management, and or monitor the effectiveness of its treatment. For some superficial skin and yeast infections, a clinical examination of the patient combined with microscopic examination of the sample may be sufficient to determine that fungal disease is present, even if the specific fungal pathogen is not identified. For deep- seated and systemic infections, a combination of diagnostic tests may be required in order to obtain a definitive diagnosis. These include microscopy to detect fungal elements, culture, detection of circulating antigens and antibodies, and molecular tests. More recently, molecular and proteomic approaches have increasingly dominated the conventional identification of pathogenic yeasts and, to some extent, filamentous fungi, since traditional methods are time consuming. More importantly, conventional methodologies have failed to identify common organisms that display uncharacteristic profiles, or fungal pathogens that are rarely encountered. The ‘gold standard’ for the definitive diagnosis of fungal disease is histology or culture of the fungal pathogen from a clinical specimen. A specimen will routinely be inoculated onto several different types of media, and then incubated at specific conditions and temperatures for up to twenty-one days. Media plates will be examined periodically for growth, and staff will try to identify the fungus using both macroscopic and microscopic morphologies. The few biochemical tests available, e.g. the urease test, can be helpful in identification, most often for yeast species. Microscopy of fungal isolates, histopathological examination of tissue, and fungal specific stains play fundamental roles in the diagnosis of infection for the variety of fungi that cause disease. The most common stain for identifying fungal elements from a cultured isolate is lactophenol fuschin/aniline blue stain. Figure 10.1 depicts the fruiting body (conidiophore) of Aspergillus fumigatus species complex, the most prevalent fungal species responsible for invasive aspergillosis (IA) in severely immunocompromised individuals. Figure 10.2 illustrates the phenotype of a three-day old colony. Serological tests are beneficial when non-culture based diagnosis of fungal disease is required. Complement fixation is predominantly used to diagnose endemic mycoses, e.g. coccidioidomycosis, blastomycosis, and histoplasmosis.