Allergy to ustekinumab: Validating skin tests for diagnostic and therapeutic decision-making.

Monoclonal antibodies have gained prominence in treating cancer and inflammatory diseases, but their increased use is linked to rising hypersensitivity reactions. Our case report focuses on a 32-year-old female with Crohn's disease treated initially with adalimumab and later with ustekinumab. Despite ustekinumab's generally safe profile, the patient developed increasingly severe mucocutaneous reactions. Intradermal skin testing with a 1:10 (0.5 mg/ml) concentration, validated with negative controls, revealed a type I hypersensitivity reaction to ustekinumab. The patient's continuous need for the drug led to a desensitization protocol, with a generalized mucocutaneous reaction during the second cycle. This case report underscores the growing concern of monoclonal hypersensitivity and the need for accurate diagnosis and therapeutic adjustments. Skin testing, especially for type I and type IV phenotypes, is valuable but lacks standardized concentrations and accessibility. This report validates nonirritating concentrations for ustekinumab in skin testing for IgE-mediated reactions, a novel contribution to existing literature. Skin testing aided risk stratification and led to the development of a desensitization protocol. A broader application of these skin test concentrations, combined with in vitro testing, could enhance diagnostic accuracy and risk prediction for ustekinumab reactions, presenting skin testing as a promising diagnostic and stratification tool for future use.