KOH Preparation in Dermatology: Uses, Procedure & Diagnostic Value

Potassium hydroxide (KOH) prep is a rapid, office-based diagnostic technique using 10–20% KOH to dissolve keratin, revealing fungal elements, parasites, or cellular detail.

Primary Diagnostic Uses

1. Superficial Fungal Infections: Skin scrapings (tinea corporis, cruris, pedis), nail clippings (onychomycosis), hair plucks (tinea capitis). KOH clears keratin; hyphae, arthroconidia, yeast (budding + pseudohyphae) visualized under 10–40×. Add DMSO for faster clearing; Chicago Sky Blue or Parker ink enhances contrast.
2. Parasitic Infestations: Scabies—skin scraping reveals mites, eggs, feces (scybala). Pediculosis—hair shafts show nits/lice.
3. Non-Infectious Skin Lesions: KOH can soften hyperkeratotic lesions (warts, corns, calluses) for debridement or biopsy prep.
4. Nail Softening: 20–30% KOH soak (20–30 min) softens dystrophic nails for trimming, culture, or topical penetration.
5. Dermatopathology: KOH clears epidermal tissue in biopsy sections, enhancing visualization of fungal elements in periodic acid-Schiff (PAS) or GMS stains.

Procedure Essentials

• Collect specimen at active border (skin) or subungual debris (nail).
• Place on slide, add 1–2 drops KOH, coverslip.
• Gently heat (flame or warmer) 30–60 sec—accelerates keratinolysis; avoid boiling.
• Examine systematically: start low power (10×) for overview, confirm at 40×.

Limitations

• False negatives: insufficient sample, prior antifungal use, non-viable organisms.
• Does not identify species—culture/PCR needed for speciation.
• Caustic—protect skin/eyes; neutralize spills with acetic acid.

KOH prep remains the cornerstone of point-of-care fungal diagnosis—fast, cheap, and highly sensitive when performed correctly.