Griseofulvin-resistant dermatophytosis correlates with in vitro resistance.

The unsuccessful treatment of dermatophytosis with griseofulvin is common. The mechanism is not known but may involve infection with a griseofulvin-resistant dermatophyte. The mean minimal inhibitory concentration (MIC) value of griseofulvin for Trichophyton rubrum isolates obtained from griseofulvin unresponsive patients was substantially larger than the mean MIC value for responsive control isolates. This difference indicates that therapeutic failure does correlate with the relative in vitro resistance. An MIC of 3.0 microgram/mL or greater was determined to indicate relative griseofulvin resistance. We conclude that the MIC determination for griseofulvin can be used to determine the appropriateness of griseofulvin therapy.