KOH mount for fungi

Principle

• Potassium hydroxide (10–20%) digests keratin, cellular debris and proteins, while fungal cell walls (chitin, glucans) resist digestion.
• Clearing the specimen makes hyphae and yeast cells visible.

Specimen

• Skin scrapings from the active edge of a lesion after cleaning with 70% alcohol.
• Nail clippings and subungual debris; plucked hairs including the root.
• Other specimens such as tissue, corneal scrapings or fluids.

Reagents and equipment

• 10% KOH for skin and hair, 20% or stronger for nails; optional dimethyl sulfoxide (DMSO) to speed clearing.
• Optional calcofluor white (binds chitin, viewed under fluorescence) or Parker ink for contrast.
• Slides, coverslips, microscope.

Procedure

• Place the specimen in a drop of KOH on a slide and add a coverslip.
• Let it clear at room temperature for about 10–30 minutes (nails may need longer); gentle warming speeds clearing but do not boil.
• Press the coverslip gently to spread material.
• Examine with 10× objective and reduced light, then 40× for detail.

Results

• Dermatophytes: septate, branching hyphae, sometimes breaking into arthroconidia; hair may show ectothrix or endothrix invasion.
• Candida: budding yeasts with pseudohyphae.
• Malassezia (pityriasis versicolor): short hyphae and round yeasts ("spaghetti and meatballs").
• Aspergillus: narrow septate hyphae with acute-angle (about 45°) branching; Mucorales: broad, ribbon-like, pauciseptate hyphae with wide-angle branching.

Quality control and pitfalls

• Mosaic artifact (lipid or cholesterol deposits along cell borders), cotton or synthetic fibers and cell membranes can mimic hyphae.
• Dried preparations form KOH crystals; examine promptly.
• A negative KOH does not rule out infection; always culture (e.g. Sabouraud dextrose agar).

Clinical use

• Rapid bedside or laboratory diagnosis of tinea, onychomycosis, candidiasis, pityriasis versicolor and some deep fungal infections.