• 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
• 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.
• consists of a central area of superficial skin edema (wheal) surrounded by erythema (flare). 
• Most penicillin-allergic patients are not allergic to the intact penicillin molecule. The majority of patients are allergic to degradation products of penicillin covalently linked to self proteins. 
• Thus, skin testing with a simple solution of the drug as it is normally administered is not adequate to identify allergy in most patients

Procedure

• Skin testing should not be performed in the weeks immediately following an episode of anaphylaxis, because during this type of severe allergic reaction, there is massive activation of mast cells, and the cells may then be hyporeactive for a period of time.
• customary to allow a period of at least four weeks to elapse after anaphylaxis 

References:

1. Skin test concentrations for systemically administered drugs – an ENDA/EAACI Drug Allergy Interest Group position paper
2. Guidelines for performing skin tests with drugs in the investigation of cutaneous adverse drug reactions