Fungal cultures are indicated when clinically significant fungal infection is suspected. Symptomatic fungal infections commonly can be characterized as follows:
Fungal cultures are used as the most sensitive routine laboratory method for detection of fungal infections.
Method
The media inoculated vary, depending on the specimen submitted and type of pathogen suspected.
Direct examination, such as wet mount or calcofluor white staining, should be performed for most specimen types; see Fungal Wet Mount. Specimens for routine fungal culture are inoculated onto nonselective media, such as BHI or Sabouraud dextrose " BHI agar. For specimens likely to be contaminated, selective media, such as inhibitory mold agar, are inoculated. An enriched medium, such as BHI blood agar, is inoculated to improve recovery of dimorphic fungal pathogens.
Special media may be inoculated for some types of specimens or suspected pathogen, like Bird (niger) seed agar for Cryptococcus neoformans, chromogenic agar for differentiation of Candida isolates, or dermatophyte test medium for dermatophytes. If Malassezia furfur is suspected, medium supplemented with a source of long-chain fatty acids (like olive oil) is inoculated.
Inoculated media are typically incubated at 25 " 30 °C in room air for up to 4 weeks. Cultures for isolation of systemic, dimorphic pathogens may be incubated at 35 " 37 °C, but the incremental yield, versus 30 °C incubation, is minimal. Cultures for fastidious pathogens are incubated for up to 8 weeks.
Media inoculated for aerobic bacterial culture will support the growth of the common yeast pathogens, Candida species, so specific culture for yeast is not usually required.
See Blood Culture, Fungal for information regarding the detection of fungemia.
Turnaround Time
Cultures for yeast are incubated for 7 days. Routine fungal cultures are incubated for up to 4 weeks. Cultures for systemic dimorphic pathogens are incubated for up to 8 weeks. Additional time is needed for isolation and identification of isolates.
Special Collection and Transport Instructions
Specimens are collected using sterile technique and transported in a sterile container within 2 hours. Store specimens at 4 °C if transport will be delayed.
Most specimens for fungal culture are collected following standard specimen collection instructions.
Collection of specimens using swabs is not recommended, except for samples from mucous membranes for the diagnosis of candidiasis.
SPS anticoagulation is recommended for blood and bone marrow specimens.
Pluck multiple hairs (10 or more) and scrape scalp from involved areas.
Wipe affected nail with 70% alcohol. Submit nail clippings and scrapings from beneath nails in a clean container.
Wipe affected skin lesions with 70% alcohol. Scrape the advancing margin to remove superficial cells and keratinized material; submit in a clean container.
Interpretation
Expected results: No growth.
Positive: Positive cultures must be carefully interpreted to ensure that endogenous fungal flora and culture contaminants are recognized.
Limitations
The results of fungal cultures may not be available when decisions regarding therapy of acute infection are required. Empiric therapy may be required.
Common pitfalls: Isolation of endogenous Candida species or environmental mold contaminants may result in unneeded treatment.
Other Considerations
Clinical information, such as travel history, immune status, and animal exposure, should be included on the requisitions for fungal cultures.
Histopathologic testing and immunologic testing are important methods for diagnosis of invasive fungal infections. Specific molecular diagnostic testing shows promise for sensitive and specific diagnosis with short turnaround time.
A diagnosis of vaginal and oral candidiasis (thrush) can be made reliably by direct microscopic examination (Gram stain or wet mount) of scrapings of mucosal surfaces without need for fungal culture.
Detection of antigen or fungal products, such as cryptococcal antigen, histoplasma antigen, ²-d-glucan, or galactomannan, may be useful for diagnosis.
The India ink wet mount is less sensitive than cryptococcal antigen testing for meningitis caused by C. neoformans.