patchtest

Patch testing is a diagnostic procedure used to identify substances that trigger allergic contact dermatitis. It involves applying a standardized set of allergens to a person’s back under occlusion for a defined period, typically 48 hours, with follow-up readings at around 48 and 96 hours (and sometimes later) to detect positive reactions. The goal is to determine which specific substances the patient cannot tolerate, so avoidance strategies can be developed.

The test uses commercially prepared panels that include common contact allergens. Widely used panels include the European Baseline Series and the North American Standard Series, as well as specialized subsets for metals, fragrances, preservatives, plastics, and personal care products. The test is usually performed by a dermatologist, allergist, or occupational medicine specialist.

Preparation and interpretation are important. Patients are usually advised to avoid topical steroids on the back for 1–2 weeks before testing and to inform the clinician of any medications that could affect results, such as systemic immunosuppressants. The patches are applied and left in place for 48 hours; a first reading is taken at removal, with a second reading 96 hours after application. Reactions are documented as negative, irritant, or positive, with a grading scale describing intensity. Positive reactions suggest sensitization to the tested allergen, guiding avoidance and management; negative results do not exclude contact dermatitis and may require further testing or repeat evaluation.

Limitations include false positives from irritant reactions, false negatives due to active dermatitis or recent corticosteroid use, and incomplete allergen panels. Patch testing is most informative when performed on stable skin and interpreted in the context of the patient’s history and exposures.