KOH Prep (Nail) — Onychomycosis

Test Characteristics

Metric	Value
False-negative rate	39% (sensitivity 61%)
False-positive rate	5% (specificity 95%)
Bayes factor (positive test)	10×
Bayes factor (negative test)	1/2×
Base rate	50% of patients presenting with dystrophic nails

Interpreting Results

Scenario	Prior	+ Result	− Result
Dystrophic nail	50%	10 × 50% ≥ 100%50%× 10100%	1/2 × 50% = 25%50%÷ 225%
Diabetic with dystrophic nail	60%	10 × 60% ≥ 100%60%× 10100%	1/2 × 60% = 30%60%÷ 230%

10 × 50% ≥ 100%50%× 10100%: exact posterior is 91%. 10 × 60% ≥ 100%60%× 10100%: exact posterior is 94%. A positive KOH is strong confirmation — if you see hyphae, it's fungal. But KOH misses nearly 40% of true onychomycosis, so a negative result is weak (1/2×). Only about half of dystrophic nails are actually fungal; the rest are psoriasis, trauma, lichen planus, or aging. If KOH is negative but clinical suspicion is high, consider fungal culture or PAS staining of nail clippings (more sensitive).

• + result: at a coin-flip prior (50%), risk pushes to 91% — strong confirmation
• − result: only drops to 25% — still a 1 in 4 chance despite negative KOH

Sources:

• Meta-analysis, BMC Infectious Diseases, 2017. PMC 5320683.
• AAFP. Onychomycosis: diagnosis and treatment.
• AAFP. Onychomycosis: rapid evidence review.

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