Contrast Allergy and Shellfish


shrimp_cocktailA recent EMedHome Clinical Pearl sheds some light on the alleged relationship between “allergies” to radiocontrast/iodine and seafood allergies.

The pearl noted that iodine is found throughout our bodies and is added to most kinds of table salt used in the United States. Our thyroid glands need iodine to function properly. While seafood contains iodine, the allergies to seafood are due to muscle proteins, not to the iodine.
Because reactions to IV contrast are not IgE-mediated, they are not considered “anaphylactic” or “allergic.” Sensitization does not occur since the reactions are not immune-mediated. In other words, your immune system won’t “remember” a prior reaction to contrast material.
Administration of steroids has no effect on whether a severe reaction will occur. Since the reaction is not “allergic”, Benadryl probably won’t have any effect, either – although this was not specifically stated in the study.
Severe reactions to contrast media occur in 0.02-0.5% of cases and deaths occur in 0.0006-0.006% of patients (something else to consider when deciding whether to undergo repeated CT scans), but serious reactions and death are not related to allergies to iodine/seafood or to prior reactions to contrast media.

One recently-published study used to create the pearl dispels this “medical myth” quite nicely.


  1. Cool video explaining allergic reactions WC. It seems counter intuitive though that your body would damage itself with histamine trying to protect itself from repeat allergens.

  2. You know, I had a reaction to contrast dye. Im not allergic to shellfish. I did take a antihistamine, I still had new crops of rash for 3 days after.

    • Good question. This article explains it better than I could do, but it seems to be an anaphylactoid reaction to the hypertonicity of the contrast material. Nonionic contrast agents such as Isovue have 1/3 to 1/4 of the tonicity of the older agents, so the incidence of reactions to these agents is much less.
      So how’s school going?

      • Huh, interesting article. I guess I didn’t realise that you could have the massive degranuation of mast cell/basophils without the IgE activation.

        School is fascinating. Having been a nurse is a blessing and a curse at times. A blessing in that at least 2 or 3 times a day I have massive ‘aha!’ moments when I finally understand the deeper reasoning behind something. And a curse in that I am an emergency nurse! I want to be DOING things. Sitting around and talking about management of acute M.I’s is like talking about executing a complex defense strategy but never getting to step on the field to play that game.

        I tell myself ‘patience grasshopper’ I will have narc seekers screaming at me again soon enough. 🙂

  3. LibraryGryffon on

    Which leads me to one other, slightly off-topic question. Why is it always assumed that if you are allergic to one type of shellfish, you are allergic to all of them? My grandmother had anaphalaxis with lobster but could eat shrimp. I have a strong asthmatic reaction to both lobster and scallops but shrimp and crab are fine.

    • Good question that I don’t know the answer to. You just need to explain to people the things you can and cannot eat – although from a medical standpoint, I don’t think you’re going to get lobster served to you in a hospital.

      • I wondered that too. I am allergic to ALL invertebrate sea dwellers as best I can tell – although I have never eaten a Sponge or an Echinoderm so who knows about them. However, I am much less allergic to mollusks than to crustaceans. What I have always wondered also is am I allergic to insects? Not that I have the motivation to eat those grasshopper tacos they sell in Mexico.

  4. On Monday I was at the hospital with my Brother-in-law, as they wheeled him away for the CAT scan, I heard the technician asking about seafood and shellfish allergies.

    My mother and sister had a baffled look on their faces and thought maybe the technician thought his migraine was allergy induced.

    I felt smart when I explained he was going to be getting contrast and the technician was just trying to gage the reaction possibilities.

    If this study is accurate, there is little point to asking those questions to patients. But, because of lawsuits, if we don’t ask these types of questions, juries will find us guilty because they can’t believe that this kind of stuff just happens.

    I wonder is there is a genetic marker that might indicate someone is more likely to suffer a contrast reaction

    • If the reaction is IgE-mediated, then previous reactions would be a gauge. With contrast allergies, the reaction is caused by the dye itself, so there’s no warning. I’d be surprised if there was a way to test for such reactions.

  5. I am allergic to shellfish and have no problem with contrast media. However, I know some people that have had reactions to it (and were not allergic to shellfish) and then needed subsequent studies – so they got pretreated with steroids and they had no reaction. Coincidence or what?

    • Our house uses Isovue for everyone now – even though it is more expensive. I haven’t seen/heard of a reaction to contrast dye in our hospital in many years.

  6. About the only type of seafood I really like is the shellfish variety, so it would be a sad day in my house if I were allergic to it. Although in truth I guess it would just mean I would have to start eating a different type of fish 😉

    As Don said, this really could change how these types of allergies are addressed…

  7. appreciate the link. i’ve been preaching this one to the fossils in our radiology department for months. they have a bunch of antiquated rules that they just won’t budge on.

    another one that drives me bonkers is their refusal to move trauma patients to the ct scanner without doing plain films of the c-spine first. if you want to hear me yelling at radiology, send me an obtunded trauma victim for whom i want to scan the head and neck.

      • yeah.. let me know when you publish a case report about babies with birth defects because i did a toe x-ray on pregnant mom

      • Don’t even get me started. 27 yo F with peritoneal abd. Surgeon won’t come in w/o CT. Tech won’t scan w/o negative preg test. Blood test machine “down” and pt can’t pee. Finally foleyed (going to OR anyway) over nursing objections (why foley for HCG when married patient tells you she hasn’t had relations in 3 months), of course negative preg (as the patient told us in the first place) and a perfed appy.

        Outcome was good, but if it wouldn’t have been, it would, of course, been my fault for the delay. When did we give techs the ability to over-rule MDs?

      • Different hospitals, different practices. I just need to know that the patient isn’t pregnant before I irradiate their abdomen. Sometimes what the patient tells me is enough to convince me. Sometimes not. If I’m refusing to do an xray because I think the patient may be pregnant I’m not overriding the medic, I’m doing my job.

    • I’m an x-ray tech,and would prefer to CT the neck straight off rather than x-ray. If you are going to CT a neck then you don’t need plain films.

  8. I was asked if I was allergic to shellfish when having IVPs and I don’t recall about the CTs.

    But WC, are you also saying that even if a pt never had a reaction to any previous radiologic testing ..they still could with another test?

  9. Great info. I reacted to xray contrast many years ago. I also became allergic to shrimp shortly thereadter. I am happy to learn that it isn’t the iodine in both cases and will celebrate your information with a bag of salty peanuts

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