Shellfish Allergies and Radiocontrast

Last month, I was paged by a nurse that a patient scheduled to receive radiocontrast had seafood allergies.  Could the patient still receive radiocontrast?  My gut instinct was to say, "yes."  But, regarding the common perception of a link between shellfish allergies and radiocontrast allergy, is there any data to support the claim?

Paper: Schabelman, E., and Witting, M. The Relationship of Radiocontrast, Iodine, and Seafood Allergies: A Medical Myth Exposed, J. of Emer. Med. (2009).

Objective: In a survey of Midwestern medical centers, 2/3 of radiologists and 89% of cardiologists ask about shellfish allergies prior to giving contrast.  Also, 35% of radiologists and 50% of cardiologists deny patients contrast or pre-medicate patients prior to giving contrast to patients with shellfish allergies.  The paper is a systematic review of data estimating the risk of contrast allergy in patients with shellfish allergy.

Result: There is actually only one study which examines the rate of allergic reaction to radiocontrast in patients with shellfish allergies - it was done in 1975.  The study revealed that the rate of allergic reaction to radiocontrast is no higher in those with shellfish allergies when compared to patients who have any allergies in general (including food allergies and asthma).  In general, atopy confers an increased risk of reaction, but people with shellfish allergies are at no higher risk.  Even in patients with atopy, the risk of a severe allergic reaction to modern radiocontrast is pretty low - estimated at about 0.05%.  Most cases of allergies to radiocontrast are pretty mild.

Conclusion:  Shellfish allergy does not especially increase the rate of allergy to radiocontrast.  A general history of allergy should suggest increased susceptibility to allergic-type reactions, but in most cases, the reaction is pretty mild.  Also, from a molecular standpoint, there is no clear relationship between shellfish allergy and radiocontrast.  Most patients who are allergic to shellfish are actually allergic to the tropomyosin in shellfish, which is a protein for muscle contraction and unrelated to iodine!  Finally, even in those with "allergies" to radiocontrast, the reaction is not actually mediated by sensitized IgE antibodies.  The reaction to IV contrast is anaphylactoid as opposed to anaphylactic (true IgE mediated reaction).  In anaphylactoid reactions, mast cells degranulate as a result of direct stimulation rather than IgE immune triggering.  A clinical consequence of this immunological fact is that someone with a previous mild allergy will likely continue to have mild allergies since the phenomenon does not depend on titers of sensitized IgE antibodies or immune memory.