Fungi cause a wide variety of diseases in the critically ill patient, from mucocutaneous infections to bloodstream infection and deep-seated, tissue-invasive disease. Thehighmortality of invasive fungal infections and the availability of new antifungal agents with different spectra of activity make prompt, accurate, specific diagnosis critical. When an invasive fungal infection is suspected, appropriate specimens, such as blood, body fluids, and/or tissue, should be sent to the laboratory specifically for fungal microscopic examination and fungal culture. The microscopic examination involves preparing a slide of the specimen with potassium hydroxide, a Calcofluor White stain, or a Gram stain.

There are two common morphologic forms of fungi that may be seen on microscopic examination and in the culture plate or tube: yeasts and molds (or filamentous fungi). Yeasts are fungi that reproduce by budding or, in rare cases, by binary fission. If the laboratory reports yeastlike forms on a microscopic examination, the most common organisms to consider would be Candida species and Cryptococcus species, although endemic fungi such as Histoplasma (agent of histoplasmosis), Blastomyces (agent of blastomycosis), and Paracoccidioides (agent of paracoccidioidomycosis) would also be possible, but less likely. Candida species are common colonizers of sites such as the oropharynx, gastrointestinal tract, and female genitourinary tract, but they may also cause devastating invasive infection. Cryptococcus neoformans should always be considered a pathogen. If the laboratory reports the growth of yeast in culture, the same organisms should be considered, although Histoplasma, Blastomyces, and Paracoccidoides would be much less likely since they are dimorphic fungi and more often grow as molds.