Penicillin skin testing is a tool used in the diagnosis of penicillin allergy. This topic will review the indications, safety, protocols, and interpretation of skin testing with penicillin and the aminopenicillins (amoxicillin and ampicillin).
An overview of allergy skin testing (for various types of allergens), a general discussion of the diagnosis of drug allergy, and the clinical manifestations of penicillin allergy, are discussed separately. (See "Overview of skin testing for allergic disease" and "An approach to the patient with drug allergy" and "Allergy to penicillins".)
INFORMATION PROVIDED BY SKIN TESTING
Skin testing is a bioassay, performed on the skin, which detects the presence of allergen-specific immunoglobulin E (IgE) on a patient's mast cells. When allergen is introduced into the skin of a patient during skin testing, it comes into contact with cutaneous mast cells. Binding of the allergen occurs if the patient's mast cells are coated with IgE recognizing that specific allergen. If both IgE and allergen are present in sufficient quantities, then adjacent allergen-specific IgE molecules become cross-linked on the mast cell surface and the cells are activated.
Mast cell activation results in a positive skin test, which is a transient "wheal-and-flare" reaction within 15 to 20 minutes from application of the allergen. This reaction consists of a central area of superficial skin edema (wheal) surrounded by erythema (flare). This pruritic reaction represents the immediate phase of the allergic reaction. If the patient's cutaneous mast cells are not activated, then no edema or erythema develops and the test is negative. Falsely-negative skin tests can result when patients have received medications such as antihistamines that block the skin effects of immediate mast cell mediators. (See 'Patient preparation' below.)
Patients with detectable penicillin-specific IgE on their mast cells are said to be "sensitized" to penicillin and are at high risk for an immediate allergic reaction to the drug. Immediate allergic reactions are defined as those reactions that begin within one hour of administration of a medication. (See "Drug allergy: Classification and clinical features", section on 'Definition and classification of drug allergy'.)