A new medium for the presumptive identification of dermatophytes.

A new medium, Dermatophyte Identification Medium (DIM) (trade mark pending), was specifically developed to eliminate problems of false-positive results associated with commercially marketed media, such as dermatophyte test medium (DTM). Previous investigations had demonstrated that DTM only partially suppressed growth of nondermatophytes and that several of these nondermatophytic fungi that were morphologically similar to dermatophytes caused false-positive results. Presumptive identification of an unknown isolate as a dermatophyte required only the transfer of a portion of the suspected colony recovered from the specimen to DIM. Positive results, evidenced by a change in the color of the medium, were observed within 24 to 48 h. In studies of over 500 isolates of dermatophytes and common nondermatophyte molds, as well as close to 600 yeast isolates, false-positive results were always associated with bacterial contamination of the mold isolates while false negatives were only observed with occasional isolates of Trichophyton verrucosum. DIM culture was an inexpensive, rapid, and accurate method for the presumptive identification of dermatophytes in the clinical mycology laboratory.