Diagnostic Dermatopathology and Related Clinical and Laboratory Knowledge
Core principle
Every diagnosis is a synthesis of four inputs: the clinical history, the lesion's gross and microscopic pattern, the laboratory data, and the differential that the pattern generates. Candidates must know that most dermatopathology errors are errors of omission (missing a history) or of pattern recognition (calling a pattern before it is complete).
Concepts to master
- Reaction patterns as the first organizing step: spongiotic, interface, psoriasiform, lichenoid, vesiculobullous, granulomatous, vasculopathic, panniculitic, and fibrosing.
- Clinical-pathologic correlation: drug history, duration, site, and evolution change the differential for the same histology.
- Dermoscopy and gross morphology as supplementary data (e.g., pigment network, vascular patterns).
- Laboratory data: direct immunofluorescence (DIF), serology (desmoglein ELISA, BP180/BP230), and cultures.
Traps
- A psoriasiform pattern is not psoriasis; think pityriasis rubra pilaris, lichen simplex chronicus, tinea, and secondary syphilis.
- Interface dermatitis with eosinophils suggests drug reaction; with neutrophils suggests connective tissue disease.
- Always ask whether the biopsy is representative (edge of ulcer, superficial shave of a deep process).
Worked rule of thumb
If a biopsy shows interface dermatitis, check for eosinophils, neutrophils, and deep infiltrate. Eosinophils plus deep perivascular infiltrate favor a drug eruption; neutrophils plus mucin favor lupus; plasma cells favor syphilis or a mucosal process.
Key points
- Reaction pattern first, then refine by cell type and distribution.
- History and DIF frequently change the diagnosis.
- Psoriasiform does not equal psoriasis.
- Eosinophils, neutrophils, and plasma cells are discriminating cells in interface disease.
- Ensure the biopsy represents the process.