Allergy testing is a way to identify what types of allergens--for example, food, dust, pollen or animal dander--are causing sneezing, wheezing and breathing problems in a person. Allergy testing may be performed as a blood test, an allergen challenge test or a skin prick test. The latter is most often used, and results are graded as wheals and flares to measure the body's allergic reaction. Because the allergic reaction depends on histamine release from the body, certain medications that reduce histamine levels should be stopped before allergy testing.
Antihistamines
Prescription and over-the-counter antihistamines (e.g., Claritin, Benadryl and Zyrtec) block histamine actions in the body to prevent allergy symptoms. Because of this, these drugs, as expected, impair the body's histamine response in allergy testing as well. Skin testing usually includes a positive histamine control prick, and taking antihistamines will reduce the wheal and flare response, even to the histamine prick. Doctors recommend avoiding antihistamines for at least two and up to seven days before allergy testing.
H2 Blockers
Medications like Zantac and Tagamet (i.e., ranitidine and cimetidine) are used to treat acid reflux and related stomach problems. These medications are histamine receptor-2 (H2) blockers; they also prevent histamine activity in the body, but their main activity is at the histamine receptors of the stomach instead of the skin and nose, for example. This activity suggests interference with allergy testing, and standard testing recommendations as of early 2009 included avoidance of H2 blockers before allergy testing. However, a report by K.M. Shah, et al., at the American College of Allergy, Asthma & Immunology 2009 Annual Scientific Meeting showed no significant interference of allergy testing in patients who were taking H2 blockers at the time of testing.
Tricyclic Antidepressants
The same 2009 report confirmed the interference between tricyclic antidepressants (TCAs) and skin prick allergy testing. Approximately half of the patients taking TCAs during allergy testing did not experience a reaction to the histamine skin prick. Although a mechanism is not completely understood for the reduced histamine reaction, tricyclic antidepressants also should be avoided before allergy testing for the most accurate test results.
References
- American College of Allergy, Asthma & Immunology (ACAAI) 2009 Annual Scientific Meeting: Abstract 23. Presented November 8, 2009.
- MayoClinic.com: Allergy Skin Tests: How You Prepare
