Other nail diseases may mimic a fungal infection. Therefore the diagnosis of onychomycosis requires an assessment of both clinical and laboratory features. Onychomycosis is comprised of four distinct types, each of which requires a different technique to obtain a nail specimen. A potassium hydroxide (KOH) preparation may be used to confirm the presence of fungi. However, a fungal culture is required to identify the specific genus and species of the pathogen. It may be necessary to perform histopathologic analysis of the nail unit in cases in which the clinical appearance suggests the presence of a fungal nail infection, but the KOH preparation and culture are negative. Copyright © 1996 by the American Academy of Dermatology, Inc.