Nail clipping

Role

Nail clipping for microscopy/culture is the standard investigation for suspected onychomycosis (fungal nail infection) and other nail plate pathology - confirming the diagnosis before committing a patient to prolonged systemic antifungal treatment, given the significant clinical overlap between fungal and non-fungal causes of nail dystrophy
Onychomycosis is common, but a substantial proportion of clinically "fungal-looking" nail dystrophy is actually due to another cause (psoriasis, trauma, lichen planus) - making confirmatory testing valuable before starting systemic antifungal therapy

Nail clippings (and subungual debris) are examined by direct microscopy (potassium hydroxide preparation) for fungal elements, and sent for fungal culture to identify the specific organism (dermatophyte, non-dermatophyte mould, or yeast) - species identification is relevant since treatment response and duration can differ by organism

Microscopy has reasonable sensitivity but culture can take weeks and has a meaningful false-negative rate - a negative culture does not fully exclude onychomycosis if microscopy was positive or clinical suspicion remains high, and repeat sampling or histological nail plate examination (PAS staining) can be considered in this scenario
Distinguishing fungal nail dystrophy from psoriatic nail disease (pitting, onycholysis, oil-drop sign) and other causes of nail dystrophy is important, since these require entirely different management

Confirmed onychomycosis -> systemic antifungal therapy (e.g. terbinafine) for an appropriate duration (often months, given slow nail growth), given topical treatment alone has limited efficacy for established nail plate infection
Unconfirmed/negative testing with a compatible clinical picture -> reconsider the differential (psoriasis, trauma, lichen planus) rather than treating empirically based on appearance alone, given systemic antifungal treatment carries its own monitoring requirements (hepatotoxicity) and cost/duration burden

Psoriatic nail disease (the key differential), dermatophyte vs non-dermatophyte mould infection, terbinafine hepatotoxicity monitoring

Confirmatory testing before starting systemic antifungal treatment is worthwhile given the significant clinical overlap between fungal and non-fungal nail dystrophy, and the meaningful duration/monitoring burden of systemic antifungal therapy - empirical treatment based on appearance alone risks both under- and over-treatment

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