Which allergy test applies a patch with potential allergens on the skin and is typically read after several hours or days?

• A. Scratch test
• B. Prick test
• C. Intradermal allergy testing
• D. Skin patch test

Answer: (D)

Testing for delayed, cell-mediated reactions to contact allergens is what the skin patch test does. In this method, small patches containing potential allergens are affixed to the skin (usually on the back) and left in place for about 48 hours. The skin is then examined, with readings typically done at 48 hours and again at 72 hours (or later) to detect delayed reactions that develop over time. This pattern—patches applied to the skin and read after several hours to days—distinguishes it from tests that measure immediate, IgE-mediated allergies, which are read within minutes to a few hours.

Other skin tests involve scratching or pricking the skin or injecting allergen solutions under the skin, and they are designed to identify immediate hypersensitivity rather than a delayed reaction, so they’re read much sooner and aren’t performed with patches left on the skin for days.