I hear it over and over again.

From a nurse, “She can’t have povidone prep, she has a shellfish allergy.”

From the patient, “I can’t have the contrast because I’m allergic to iodine and shellfish.”

The list of allergies placed in the chart by a doctor, “Radiocontrast. Iodine. Shellfish.”

It’s not the fault of the patients who are typically repeating the medical misinformation they have been given. However, the medical professionals who perpetuate the iodine-shellfish-radiocontrast allergy drive me batty because they either don’t understand the medicine involved or understand there is no link but don’t take the time to educate the patient and correct the chart.

In short, I hate medical myths. Undoing what someone once said to somebody is hard because when someone has believed something for so long (whether they are a doctor, nurse, or patient) they are not always open to a change. I estimate that it takes a minimum of three conversations to under one incorrect piece of information.

Let’s break it down.

While shellfish allergy is a very real allergy and potentially life threatening, it is not an iodine allergy. The major allergens in shellfish are tropomyosins, which are proteins is the muscle and definitely not iodine. Tropomyosins cross react among various shellfish, but not scaled fish so that is why you can be allergic to crab and eat salmon. People who are allergic to scaled fish are typically allergic to a different muscle protein, parvalbumin.

Iodine is not an allergen. We all have iodine in our bodies. It is in our thyroid hormones and in amino acids. We would die without iodine. Iodine deficiency is such a potential health problem that most table salt in the United States contains iodine. Listing iodine as an allergen in the chart is wrong. It is worth repeating, iodine cannot be an allergen.

People can have reactions to povidone-iodine prep (which contains iodine), but this is due to allergens in the solution not the iodine. If someone has a reaction to povidone-iodine prep the prep should be listed as the allergen, not the iodine. It’s an important distinction. With many products it’s the additives that drive an allergic reaction.

People can have severe reactions to radiocontrast, but these are not allergic reactions. Anaphylaxis is due to a the immune system producing IgE immunoglobulins in response to an allergen, such as the tropomyosin in shellfish. It is an allergic response. When a person is re-exposed, the allergen-IgE complex triggers the severe inflammatory cascade.

Reactions to radiocontrast are believed to be anaphylactoid and so are not caused by IgE. What’s the difference? Anaphylaxis requires IgE to trigger the inflammatory cascade and with an anaphylactoid reaction the substance directly stimulates the inflammatory cascade, no immune system intervention is needed. What causes triggers anaphylactoid reactions with radiocontrast? Not the iodine, but likely the hyperosmolarity because  hyperosmolar solutions are highly irritating. As expected, anaphylactoid reactions are much less frequent with the lower osmolar radiocontrast solutions more commonly used today.

As an aside, reactions to radiocontrast (especially the older, more hyperosmolar solutions) are increased about three-fold in people with allergies to milk, eggs, and chocolate which according to the American Academy of Asthma, Allergy, and Immunology indicates a, “general atopic disposition, rather than an iodine specific reaction.” Basically, people with food allergies have a twitchy inflammatory response system and are at higher risk for non-allergen driven reactions as well.

So let’s get rid of the medical mythology surrounding iodine, shellfish, and radiocontrast. When incorrect allergies are listed in the chart it breeds confusion and both patients and their providers need and deserve accurate health information.

Shellfish allergy isn’t iodine driven.

Iodine isn’t an allergen.

Reactions to radiocontrast  are not an allergic reaction to iodine and are unlikely to be allergic reactions at all, but rather an inflammatory response to irritating hyperosmolar agents.

The science matters.

Jennifer Gunter is an obstetrician-gynecologist and author of The Preemie Primer. She blogs at her self-titled site, Dr. Jen Gunter.