What are the lab diagnosis of dermatophytes?

Diagnostic Methods for Dermatophyte Infections:

1. Potassium hydroxide (KOH) microscopy:
Values: aids in visualizing hyphae and confirming the diagnosis of dermatophyte infection Procedure: obtain scale from the active border of a lesion, pull out several loose hairs from the affected area or, in the case of nails, obtain subungual debris. A moist cotton swab rubbed vigorously over the active border of a lesion works as well as a scalpel blade and is safer. Transfer the scale, hair, or debris to a glass slide, and add a few drops of 10% to 20% KOH. For nail material or hair, gently warm the slide. The wet-mount preparation is then examined under a microscope (×400) with back-and-forth rotation of the focus knobs. This technique aids the visualization of hyphae (branching, rod-shaped filaments of uniform width with lines of separation [septa]). In tinea capitis, the hair shaft may be uniformly coated with minute dermatophyte spores.

2. Wood’s lamp examination (ultraviolet light):
Value: generally of limited usefulness, because most dermatophytes currently seen in the United States do not fluoresce; may have value in the following situations: For diagnosing a brown, scaly rash in the scrotum or axilla: erythrasma, caused by the bacterium corynebacterium minutissimum, fluoresces a brilliant coral red, whereas tinea cruris or cutaneous candidal infections do not fluoresce.
For diagnosing tinea (pityriasis) Versicolor, which fluoresces pale yellow to white
For diagnosing tinea capitis caused by two zoophilic Microsporum species that fluoresce blue-green.

3. Fungal culture:
Value: slow and expensive, but useful to confirm the diagnosis of onychomycosis when long-term oral therapy is being considered
Procedure*: Skin, nail, or hair scrapings are sent in a sterile container for inoculation on Sabouraud’s dextrose agar by a hospital or reference laboratory. The culture usually takes 7 to 14 days to be declared positive; it must be held for 21 days to be declared negative.

4. Skin or nail biopsy:
Value: may guide treatment decisions when the diagnosis is difficult to establish, a dermatophyte infection has not responded to previous treatment, or KOH microscopy is negative in a patient with dystrophic nails.