Positive Patch Test Reaction to Sodium Metabisulfite: Prevalence, Relevance and Clinical Implications

Background: Sodium metabisulfite (SMB) is widely used as a preservative in topical products, injectable drugs, and food. Contact allergy to SMB is increasing but the relevance of SMB positive patch test reactions and clinical implications of SMB systemic exposure remain challenging. Objectives: To assess the prevalence and relevance of SMB positive patch test reactions and to evaluate the tolerability of a local anaesthetic solution containing SMB. Methods: A retrospective study on consecutive patch tested patients with the Italian baseline series containing SMB was performed (January 2024–April 2025). In patients who tested positive for SMB, an intradermal test with SMB and a challenge with a local anaesthetic solution containing SMB were performed. Results: Among 550 patients consecutively patch tested with the Italian baseline series, 16 tested positive for SMB (2.9% [1.8%–4.7%]). The 16 positive reactions to SMB were not considered relevant. MOAHLFA index showed a significant positive association with Age > 40 (9.67 [1.249–74.89], p = 0.03). In all 16 patients, intradermal test with SMB and local anaesthetic challenge was negative. Conclusions: A high prevalence of positive patch test reactions to SMB in the absence of relevance was observed. Moreover, a positive patch test to SMB seems not to contraindicate the administration of a local anaesthetic containing SMB.