Thank you for your inquiry.
Skin testing for IgE-mediated reactions against antibiotics is performed identical to skin testing to any other allergen such as pollen, dust mite, et cetera. All of these are read at 30 minutes, and this has nothing to do with expiration of the substance being tested, but rather is due to the fact that this is the earliest effective time to read these tests. For your interest, here is a link to a package insert for the agent used to test for penicillin allergy, which will give you the details of the technique involved. This technique, as noted, is the same as used for other allergens and is not based on expiration date.
Thank you again for your inquiry and we hope this response is helpful to you.
Phil Lieberman, M.D.