A 59-year-old man is admitted with abdominal pain. He has a history of pancreatitis. A contrast CT scan is ordered. He reports a history of severe shellfish allergy when the radiology tech checks him in for the procedure. You are paged regarding what to do:
A) Continue with scan as ordered.
B) Switch to MRI scan.
C) Switch to MRI scan with gadolinium.
D) Continue with CT with contrast, give dose of Solu-Medrol.
E) Continue with CT with contrast give IV diphenhydramine.
The correct answer here is A, This patient can receive his scan and receive contrast as ordered.
The mistaken thought was that shellfish contains iodine, so allergy to shellfish was likely to portend allergy to iodine.
Allergy to shellfish is caused by individual proteins that are definitely not in iodine-containing contrast.1 Beaty et al. studied the prevalence of the belief that allergy to shellfish is tied to iodine allergy in a survey given to 231 faculty radiologists and interventional cardiologists.2 Almost 70% responded that they inquire about seafood allergy before procedures that require iodine contrast, and 37% reported they would withhold the contrast or premedicate patients if they had a seafood allergy.
In a more recent study, Westermann-Clark and colleagues surveyed 252 health professionals before and after an educational intervention to dispel the myth of shellfish allergy and iodinated contrast reactions.3 Before the intervention, 66% of participants felt it was important to ask about shellfish allergies and 93% felt it was important to ask about iodine allergies; 26% responded that they would withhold iodinated contrast material in patients with a shellfish allergy, and 56% would withhold in patients with an iodine allergy. A total of 62% reported they would premedicate patients with a shellfish allergy and 75% would premedicate patients with an iodine allergy. The numbers declined dramatically after the educational intervention.
Patients who have seafood allergy have a higher rate of reactions to iodinated contrast, but not at a higher rate than do patients with other food allergies or asthma.4 Most radiology departments do not screen for other food allergies despite the fact these allergies have the same increased risk as for patients with a seafood/shellfish allergy. These patients are more allergic, and in general, are more likely to have reactions. The American Academy of Allergy, Asthma, and Immunology recommends not routinely ordering low- or iso-osmolar radiocontrast media or pretreating with either antihistamines or steroids in patients with a history of seafood allergy.5
There is no evidence that iodine causes allergic reactions. It makes sense that iodine does not cause allergic reactions, as it is an essential component in the human body, in thyroid hormone and in amino acids.6 Patients with dermatitis following topical application of iodine preparations such as povidone-iodide are not reacting to the iodine.
Van Ketel and van den Berg patch-tested patients with a history of dermatitis after exposure to povidone-iodine.7 All patients reacted to patch testing with povidone-iodine, but none reacted to direct testing to iodine (0/5 with patch testing of potassium iodide and 0/3 with testing with iodine tincture).
Take home points:
- It is unnecessary and unhelpful to ask patients about seafood allergies before ordering radiologic studies involving contrast.
- Iodine allergy does not exist.
Dr. Paauw is professor of medicine in the division of general internal medicine at the University of Washington, Seattle, and he serves as third-year medical student clerkship director at the University of Washington. Contact Dr. Paauw at [email protected].
1. Narayan AK et al. Avoiding contrast-enhanced computed tomography scans in patients with shellfish allergies. J Hosp Med. 2016 Jun;11(6):435-7.
2. Beaty AD et al. Seafood allergy and radiocontrast media: Are physicians propagating a myth? Am J Med. 2008 Feb;121(2):158.e1-4.
3. Westermann-Clark E et al. Debunking myths about “allergy” to radiocontrast media in an academic institution. Postgrad Med. 2015 Apr;127(3):295-300.
4. Coakley FV and DM Panicek. Iodine allergy: An oyster without a pearl? AJR Am J Roentgenol. 1997 Oct;169(4):951-2.
5. American Academy of Allergy, Asthma & Immunology recommendations on low- or iso-osmolar radiocontrast media.
6. Schabelman E and M Witting. The relationship of radiocontrast, iodine, and seafood allergies: A medical myth exposed. J Emerg Med. 2010 Nov;39(5):701-7.
7. van Ketel WG and WH van den Berg. Sensitization to povidone-iodine. Dermatol Clin. 1990 Jan;8(1):107-9.