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Shellfish Allergy and Iodine Contrast Allergy

If you’ve ever had any radiographic imaging like an MRI or x-ray, chances are someone has asked you about any seafood or shellfish allergies. It’s a commonly held misconception that seafood allergies are linked to an iodine allergy, placing you at greater risk of an adverse allergic reaction to iodine contrast media used in imaging studies. Despite little or no evidence to support this myth, many providers continue to perpetuate poorer patient outcomes by either withholding necessary imaging or ordering premedication that increases costs, delays imaging, and can even lead to adverse effects.

This myth stems from the false belief that iodine content is a common cause of adverse reactions to both contrast media and shellfish. It started with a study published in 1975 that noted of those who had an adverse reaction to contrast media, 15% had an existing allergy to shellfish. The author attributed this to iodine in both fish and contrast media that acted as the allergen. However, the author failed to acknowledge that allergies to other foods such as chocolate and milk occurred at pretty much the same rate.

In fact, iodine is not an allergen at all. An extensive review found little to no evidence that iodine or iodide acts as an allergen, and refuted the belief that iodine-containing products triggered allergic reactions in those thought to have an iodine allergy. Anaphylactic reactions to shellfish are caused by proteins called tropomyosin, which are structurally unrelated to iodine.

Although contrast media can cause a variety of reactions, the mechanisms are still not fully understood and are very likely not due to an iodine allergy. If you’ve ever had a reaction to contrast media, it’s probably related to the physiochemical properties of the media and not the iodine content itself.

So, if you have a history of a seafood allergy, you should not have to worry about being at a higher risk for anaphylaxis from iodinated contrast media. Similarly, if you’ve had anaphylaxis from contrast media, it doesn’t mean you are allergic to seafood as well. If you have concerns about either allergy, consult with a board-certified allergist before any imaging involving contrast media.

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