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Medication Allergy Testing and Challenges

There are many different types of adverse reactions to medications. However, not all adverse reactions are allergic reactions. An allergic reaction to a medication is caused by an immune system that has an extreme “over-reaction” to that particular medication. This type of reaction can involve Immunoglobulin E (IgE) and other antibodies and/or specific cytotoxic immune cells in response to an otherwise harmless substance in the medication. One characteristic of all drug allergies is that similar symptoms will occur every time soon after the offending medicine is taken.

Evaluation

Every medication/drug allergy diagnosis should begin with a detailed medical history and physical examination. The doctor will ask many questions about the reaction(s) you experienced. This includes, where and when it occurs, how much medication was taken, what if anything was required to treat the reaction, etc. Since the drug allergy may be genetic, there will likely be some questions about other family members who may also be allergic.

Testing

  • Allergy skin testing may be performed to check for presence of allergic antibodies to selected drug allergens
  • Blood testing is occasionally indicated
  • A drug challenge to the suspected drug is sometimes necessary to confirm the diagnosis. If indicated, drug challenges are coordinated with safety as the highest priority.

Preparing for the Test

Please follow the directions described under the Allergy Skin Testing section with regards to stopping medications.

During the Test

Testing for medication allergy usually starts with prick skin testing to the medication. If there is a positive reaction to the skin test, testing stops immediately. A positive reaction indicates that you are most likely allergic to that medication. If skin testing is negative, you will then be given the medication (the challenge portion of the evaluation) by mouth starting with an extremely small dose and gradually increasing the dose over time until the desired, full dose of the medication has been consumed. If at any time signs and symptoms of a reaction are detected, the testing/challenge stops and you will be treated appropriately. If no reaction is seen, it is highly unlikely that you have an immediate, potentially life-threatening type of allergy to that medication. Your doctor will discuss the results with you before you leave the center.

On a side note, if the reaction you experienced involved an injected or topical medication, this testing process will likely be different and may involve more skin testing and/or a challenge phase involving injected or topical forms of the medication.

Length of the Test/Challenge

The skin testing part of the challenge is relatively fast and takes about 20 minutes. Penicillin testing skin testing takes a little bit longer. To complete the testing, you are then given the oral challenge. As noted this is done in several steps. The amount of medication per dose, the number of doses, and the timing of the doses is very dependent on your reaction history and could take several hours. Whether you complete the entire oral part of the challenge or whether it is stopped because of a reaction, expect to remain in center for about 1 hour after the final dose is given.