Dermatopathology and Diagnostic Techniques
Histological patterns
Learn the algorithmic patterns and their differentials. Spongiosis = eczema. Psoriasiform = psoriasis (regular acanthosis, parakeratosis, Munro microabscesses, thinned suprapapillary plate). Lichenoid/interface = lichen planus, lupus, drug eruption. Granulomatous = sarcoid, tuberculosis, leprosy. Leucocytoclastic vasculitis = fibrinoid change with neutrophils. Adding the dominant inflammatory cell (eosinophils, plasma cells, neutrophils) narrows things further.
Immunofluorescence and special tests
Direct immunofluorescence: pemphigus vulgaris shows intercellular IgG ("chicken wire"); bullous pemphigoid shows linear IgG and C3 at the basement membrane; dermatitis herpetiformis shows granular IgA in dermal papillae. Indirect titres track pemphigus activity. KOH microscopy for hyphae and spores, Tzanck smear for herpes group, Wood's lamp for selected tinea and pigment disorders.
Patch testing
The gold standard for allergic contact dermatitis. Use a standard series with readings at day 2 and day 4/5 - the delayed reading is essential. Avoid testing on active dermatitis and remember that oral steroids and potent topical steroids at the test site suppress reactions.
Biopsy technique
Choose an early, untreated, fully developed lesion, include subcutaneous fat for inflammatory disease, orient the specimen, and take multiple specimens when uncertain. A shave is adequate for exophytic tumours but not for suspected melanoma or panniculitis.
Traps
- Reading patch tests only at 48 hours and missing late positives.
- Accepting a "non-specific dermatitis" report after an inadequate superficial biopsy.
- Treating a negative KOH as proof there is no fungal infection.
- Correlating histology with a clinical history the pathologist never received.
Rule of thumb
Give the pathologist one sentence: site, duration, and your top two differentials - the same section looks different under different clinical questions.