Can I give contrast to a patient with a shellfish allergy?

The short answer: yes!

There is a longstanding theory that because shellfish has a lot of iodine in it, patients who have anaphylactic reactions to shellfish should not be given iodinated radiocontrast because that will set off anaphylaxis as well. This is kind of hand-wavy pathophysiology, as Jennifer Gunter notes on KevinMD: the substance patients do react to in contrast is the hyperosmolar agent in contrast, which tends to be irritating, and there are now low-osmolar alternatives. Iodine itself is an element in the periodic table that is in fact, essential for life, so saying you have an iodine allergy is like saying you’re allergic to water. Which, hopefully, you’re not.

This is an entertaining piece on Clinical Correlations that gives a great evidence-based explanations for why it is absolutely okay to give contrast. A review in the Journal of Emergency Medicine reports that severe reactions (the kind we care about) only occur in 0.02-0.5% of patients, and that patients with higher risk of any kind of reaction tend to be more atopic, that is, have asthma, multiple food allergies, etc.

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