For an allergy testing, Prick (epicutaneous) testing is done first (One drop of reagent is used for the prick tests;), and results are read 15 to 20 minutes later. If the prick tests are negative, intradermal testing follows (0.02 mL is used for the intradermal tests.). The skin test is positive if a wheal larger than 3mm is formed. Of patients with a negative skin test, 1% to 4% will develop non-life-threatening allergic reactions.
After an allergic reaction, the chance of having a positive response on skin testing diminishes with time.
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