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MCI/MI Allergy — SCE Dermatology MCQ

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HardInflammatory DermatosesMCI/MI AllergySCE Dermatology

Patch testing is positive to the standard methylchloroisothiazolinone/methylisothiazolinone mix. The patient uses an “MI-free” water-based paint that lists methylchloroisothiazolinone. What avoidance advice is appropriate?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: B — Avoid MCI, MI and MCI/MI in paint and personal-care products during assessment

Explanation lettering: B = shown as A · D = shown as B · A = shown as C · E = shown as D · C = shown as E

D is correct. A positive MCI/MI patch test requires ingredient-based counselling, not reliance on front-label claims. MCI remains relevant even when a product is marketed as MI-free, and water-based paints are a recognised hidden occupational or domestic exposure. Relevance should be linked to timing and distribution, with exposure reduction and appropriate protective measures. Drying does not retrospectively make wet handling safe, topical treatment cannot compensate for ongoing allergen exposure, and unrelated fragrance or formaldehyde avoidance does not address the demonstrated sensitisation.

Reference: British Society for Cutaneous Allergy MCI leaflet: https://cutaneousallergy.org/pils/methylchloroisothiazolinone-mci/