Dermatopathology and Diagnostic Techniques
Core concepts
You need a systematic approach to H&E interpretation: low-power architecture (epidermal contour, dermal infiltrate pattern - perivascular, interstitial, nodular, lichenoid, psoriasiform, spongiotic) before high-power cytology. Learn the vocabulary of epidermal change (acanthosis, hyperkeratosis, parakeratosis, spongiosis, exocytosis), dermal change (collagen alteration, mucin, necrosis), and infiltrate composition (neutrophils, eosinophils, plasma cells, atypical lymphocytes).
Diagnostic techniques are tested as when to use what: dermoscopy for pigmented and vascular lesions (pattern analysis, ABCD of dermoscopy, blue-white veil, pigment network), patch testing for type IV allergy with correct reading at 48 and 96 hours, direct immunofluorescence for bullous and vasculitic disease, KOH and fungal culture, Tzanck smear, and molecular tests (PCR, clonality studies, FISH).
Traps
- Confusing spongiotic (eczema) with psoriasiform (psoriasis) when both show acanthosis.
- Forgetting that direct immunofluorescence is read on perilesional skin in bullous disease.
- Patch test reading: a reaction may peak late; single early reading misses positives.
- Dermoscopy: confusing a pigment network (melanocytic) with a pigment pseudonetwork (facial lentigo).
Rule of thumb
Describe first, diagnose second: architecture → cytology → special stains. If two diagnoses share a pattern, the differentiating clue is usually a single feature (e.g., neutrophils in stratum corneum favour psoriasis over eczema).