Drug Eruption — SCE Dermatology MCQ

Correct answer: D — Lymphocyte transformation test (drug-specific)

The correct answer is D, Lymphocyte transformation test (drug-specific). This patient has a delayed, T-cell mediated cutaneous drug reaction (morbilliform eruption with interface dermatitis and eosinophils, plus deranged liver function, a DRESS-spectrum pattern) with two concurrent candidate drugs, methotrexate and allopurinol. The LTT co-incubates the patient's peripheral blood mononuclear cells separately with each suspected drug and measures antigen-specific T-cell proliferation, allowing the culprit to be distinguished when multiple agents could explain a delayed hypersensitivity reaction. Biopsy confirms the type IV mechanism but cannot identify which drug is responsible, which is exactly the gap LTT fills. Allopurinol is a recognised trigger of DRESS and severe delayed cutaneous reactions, so establishing causality matters before any decision on future rechallenge or avoidance. Why the other options are wrong: E. Radioallergosorbent test (RAST): Detects drug-specific IgE and is relevant only to immediate, IgE-mediated (type I) reactions such as urticaria or anaphylaxis, not delayed interface dermatitis. A. Total IgE: A non-specific marker of atopy or parasitic infection; it cannot identify a causative drug and has no role in delayed reaction causality. B. Skin prick testing: Assesses immediate IgE-mediated sensitisation and is inappropriate for a delayed morbilliform eruption; it does not test the T-cell mechanism involved here. C. Tryptase: Rises acutely with mast cell degranulation to confirm anaphylaxis has occurred, giving no information on which drug caused a delayed, non-mast-cell-mediated eruption. Key point: When a delayed, T-cell mediated drug eruption occurs with multiple candidate drugs, the lymphocyte transformation test attributes causality to the specific agent, unlike IgE- or mast cell-based tests which apply only to immediate hypersensitivity.

Reference: BSACI Guideline for the Management of Drug Allergy (Mirakian et al., Clinical & Experimental Allergy, 2009), section on in vitro investigations including lymphocyte transformation testing for delayed drug hypersensitivity; https://onlinelibrary.wiley.com/doi/10.1111/j.1365-2222.2008.03155.x