Dermatopathology and Laboratory Diagnosis
Scope
This domain tests pattern-based histologic diagnosis, immunofluorescence, and bedside laboratory tests, including correct specimen handling and interpretation.
Core rules
- Major reaction patterns: spongiotic, psoriasiform, lichenoid, interface, vesiculobullous, granulomatous, vasculopathic, and panniculitic.
- Biopsy technique: punch or excision; avoid ulcer centers; do not use formalin for direct immunofluorescence-use Michel's medium or snap-freeze.
- DIF patterns: intercellular IgG (pemphigus), linear IgG/C3 at BMZ (bullous pemphigoid), granular IgA in papillae (dermatitis herpetiformis), linear IgA (linear IgA disease).
- Ancillary tests: salt-split skin, ELISA for desmoglein and BP180, indirect immunofluorescence, TCR clonality, flow cytometry.
- Bedside tests: KOH for hyphae, scabies prep, Wood's lamp, patch testing read at 48 and 72–96 hours.
- Histologic clues: Pautrier microabscesses (MF), Civatte bodies (lichen planus), Medlar bodies (chromoblastomycosis), ghost cells (pilomatricoma).
Worked example
A subepidermal blister shows linear IgG and C3 at the basement membrane; salt-split skin labels the roof. Reasoning: roof labeling indicates the antigen lies above the lamina densa-BP180/BP230-so bullous pemphigoid is favored.
Traps
A single biopsy from an ulcer edge may only show nonspecific inflammation. DIF on formalin-fixed tissue is uninterpretable. Patch test readings must be delayed, not immediate.