Skin biopsy

Role

Skin biopsy in the infectious diseases context is used to obtain tissue for histopathology and/or microbiological culture when infection (particularly atypical, mycobacterial, or fungal) is suspected but surface swab/scraping has been unrevealing or is inadequate for the differential being considered
Less frequently used than swab/scraping for routine skin infection, but valuable when deeper tissue sampling is needed (e.g. suspected atypical mycobacterial infection, deep fungal infection, or when histopathology is needed alongside microbiology to characterise an unusual presentation)

Tissue biopsy samples deeper dermal/subcutaneous structures inaccessible to surface swabbing, improving yield for organisms residing deeper in tissue (atypical mycobacteria, deep fungi) or where the diagnostic question requires histological pattern (granulomatous inflammation) alongside organism identification

Sample should be split for both histopathology and microbiology (including specific request for mycobacterial and fungal culture where relevant, given their need for extended incubation on specific media not covered by standard bacterial culture)
Special stains (e.g. acid-fast stain for mycobacteria, periodic acid-Schiff for fungi) on the histopathology specimen can provide a faster presumptive answer than culture, which may take weeks for slow-growing organisms

Confirmed atypical mycobacterial or deep fungal infection guides prolonged, organism-specific antimicrobial therapy, often requiring specialist infectious diseases input given the complexity and duration of these regimens
Negative culture with high ongoing clinical suspicion should prompt repeat sampling or alternative diagnostic modalities rather than excluding infection on a single negative result, given imperfect sensitivity for some slow-growing/fastidious organisms

Atypical mycobacterial skin infection, deep fungal infection, granulomatous skin disease differential (infective vs non-infective causes), extended incubation requirement for mycobacterial/fungal culture

Mycobacterial and fungal culture must be specifically requested when a biopsy is sent, since they are not automatically covered by standard bacterial culture and require different, often much longer, incubation conditions - failing to request them explicitly risks a false-negative result for these organisms

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