Patch Testing Fundamentals

Introduction to Patch Testing

The patch test is the standard diagnostic method for allergic contact dermatitis (type IV hypersensitivity mediated by T cells), according to the British Journal of Dermatology 1, 2
The patch test reproduces allergic contact dermatitis under controlled conditions in individuals previously sensitized to specific antigens, with a sensitivity and specificity of 70-80% 1, 2

The prick test detects type I hypersensitivity (immediate, mediated by IgE) associated with urticaria, allergic rhinitis, asthma, and anaphylaxis, as stated by the British Journal of Dermatology 3
The optimal readings for the patch test are made on day 2 and day 4, with an additional reading on day 7 that detects approximately 10% more positive reactions that were negative on days 2 and 4 1, 2

The patch test requires application under occlusion for 48 hours using systems like Finn chambers, where the investigator loads individual allergens into test disks mounted on adhesive tape, as recommended by the British Journal of Dermatology 1, 2
Pre-prepared systems (TRUE® and Epiquick®) are significantly more reliable than operator-prepared tests, although only a limited number of allergens are available 1, 2

The patch test is indicated in patients with persistent or chronic eczematous eruptions when contact allergy is suspected or cannot be ruled out, according to the British Journal of Dermatology 3, 4
The test is also indicated in patients with atopic dermatitis/endogenous dermatitis that has become difficult or impossible to control with the same topical treatments, as stated by the British Journal of Dermatology 5, 6
The patch test should be deferred for 6 weeks after natural or artificial UV exposure, 3 months after finishing systemic agents, and 6 months after finishing biologic agents to minimize false-negative reactions, as recommended by the British Journal of Dermatology 5, 6

The results are graded as positive, negative, or irritant, with positive reactions graded on a scale of +, ++, or +++ 1, 2
Common pitfalls include misclassifying irritant reactions as positive, and a positive test only indicates contact sensitization, with clinical relevance demonstrated through correlation with active dermatitis and sometimes confirmed with repeated open application tests 1, 2, 7