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Ask The Expert

October 7, 2013

Skin Testing for Radio Contrast Allergens

Question

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?

Answer

From the Editors: Allergists may be changing their method of evaluating patients with radio contrast reactions. New research suggests that while perhaps the majority is anaphylactoid, for which skin testing is not of value, some may actually be IgE mediated. If it is true anaphylaxis, skin testing may be of value. Read what our world experts recommend.

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. Read the entire answer.

By Prof. Pascal Demoly

I would recommend that skin testing be done immediately because 6 months have already passed since the initial reaction and it has been shown that frequency of positive test results in immediate reactors to RCM is up to 50% in patients tested within 2–6 months, whereas at later time points, the frequency of positive tests decreases significantly.

Cross-reactivity is known to occur in immediate reactions (even though less frequently than in delayed reactions), and I would therefore recommend testing with the culprit RCM as well as alternative agents.

Systemic reactions have never been described during such tests and therefore can be performed in the office.

References:

Brockow K, Romano A, Aberer W, Bircher AJ, Barbaud A, Bonadonna P, Faria E, Kanny G, Lerch M, Pichler WJ, Ring J, Rodrigues Cernadas J, Tomaz E, Demoly P, Christiansen C; European Network of Drug Allergy and the EAACI interest group on drug hypersensitivity. Skin testing in patients with hypersensitivity reactions to iodinated contrast media – a European multicenter study. Allergy. 2009;64:234-41.

Dewachter P, Laroche D, Mouton-Faivre C, Bloch-Morot E, Cercueil JP, Metge L, Carette MF, Vergnaud MC, Clément O. Immediate reactions following iodinated contrast media injection: a study of 38 cases. European Journal of Radiology 2011;77:495-501.

Pascal Demoly, MD, PhD
Allergy Unit
Hôpital Arnaud de Villeneuve - Montpellier, France

By Dr. Roland Solensky

Immediate-type reactions to RCM have traditionally been considered to be anaphylactoid, or non-IgE-mediated (also referred to as non-immunologic) mechanism. Pre-treatment regimens consisting of corticosteroids and antihistamines have been shown to be very effective in preventing the vast majority of reactions, and this is consistent with the reactions being non-IgE-mediated (such pretreatment should not prevent truly IgE-mediated reactions). Recent studies from Europe and Asia indicate that at least some immediate reactions to RCM may be IgE-mediated. This might be because nonionic media has replaced use of ionic media, and the former is less likely to result in direct non-IgE-mediated mast cell degranulation. I believe at this point, based on limited studies (some of which I summarize below), it is still unclear what proportion of reactions are IgE-mediated. Also, I am not aware of any studies in the US that have demonstrated positive skin tests with RCM.

Brockow et al (Brockow et al, Allergy 2009; 64:234-41) studied 122 patients with a history of immediate reactions to RCM. 32/126 (26%) were skin test-positive. Of those evaluated 2-6 months after the index reaction (which was felt to the ideal time to limit false negative tests performed too soon after reactions and not too late before IgE wanted), 14/28 (50%) were skin test-positive. Read the entire answer.


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