During scratch testing, what indicates a possible allergy?

The formation of a wheal within 15 minutes during scratch testing is indicative of a possible allergy. This reaction occurs when the immune system responds to an allergen by releasing histamine and other chemicals, leading to localized swelling and redness. The wheal is a raised, itchy area on the skin that appears in response to the scratch test, typically occurring quickly if an allergy is present. This swift reaction is a hallmark of an allergic response and is utilized clinically to help identify specific allergens that may be causing hypersensitivity in the patient.

While a visible rash, persistent itching, or localized swelling could also be signs of an allergic reaction, they are not as immediate or specific as the formation of a wheal. A rash may develop for various reasons, not necessarily within the time frame that indicates an allergy. Persistent itching can occur with other skin conditions unrelated to allergies, and localized swelling might not occur in everyone exposed to an allergen. Thus, the wheal's prompt formation is the primary indicator used in allergy testing.