Ask The Expert
October 7, 2013
Skin Testing for Radio Contrast Allergens
When would you recommend that the office-based practicing allergist should do skin testing for a patient requiring another study who had a moderately severe reaction?
By Prof. Sae-Hoon Kim
Based on my experience and studies of our group, positive rate of RCM skin test is quite high (over 50%) in patients with previous severe RCM hypersensitivity reaction, especially patients who experienced anaphylactic shock. Patients with previous mild reaction rarely show a positive response to the culprit RCM. I think the underlying mechanism of RCM hypersensitivity might differ between anaphylaxis (accompanied with systemic reaction) and mild-to-moderate urticaria (only cutaneous reaction). IgE-mediated immunologic mechanism seems to be involved in the pathogenesis of substantial portion of RCM-induced anaphylaxis. Non-ionic, low to iso osmolar RCM became popular these days, and the classical pseudoallergic reactions to RCM have decreased. Instead, IgE-mediated or T cell mediated immunologic reactions are increasing as the opportunity of sensitization to RCM increase due to the repeated and frequent exposure to RCM. Although more clinical and laboratory researches are needed to verify the clinical value of RCM skin testing, I think it would be appropriate to perform skin test to the patients with previous severe hypersensitivity reaction to identify the sensitization to specific RCM agent.
Not to mention, it would be the best to avoid the reuse of RCM for the patients with previous severe RCM hypersensitivity reaction. However, if another RCM-using imaging test or intervention is inevitable for them, I would recommend performing skin test for all the patients with previous severe RCM hypersensitivity reaction. Skin testing is needed more for the patients who are suspected to have IgE-mediated response to RCM, for example, patients who experienced allergic reaction after repeated exposure to RCM. But, skin test can be also positive in patients who experienced allergic reaction at the first exposure.
When the skin test is performed, the agents should include the one to which the patients reacted and the alternative ones. The positive predictive value of RCM skin test is uncertain at this stage, but the specificity of RCM skin test was quite high based on the previous studies. Thus, I would recommend avoiding RCM agents with positive response as much as possible and selecting the alternative one with negative skin test response.
Sae-Hoon Kim, MD
Department of Internal Medicine and the Institute of Allergy and Clinical Immunology
Seoul National University College of Medicine – Seoul, Korea
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