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A contrast allergy note on the chart can trigger delays, unnecessary steroids and avoided scans, yet often these labels are not clinically justified. A new practice article in CMAJ, “Hypersensitivity reactions to iodinated contrast media,” examines how the labels arise and why the standard premedication rests on contrast agents that are no longer used.

Dr. Samira Jeimy, an associate professor and program director of the division of clinical immunology and allergy at Western University and an author of the article, separates adverse reactions into Type A, the dose-dependent flushing, nausea and vasovagal symptoms anyone can get, and Type B, the bizarre ones from anaphylaxis to delayed reactions such as Stevens-Johnson syndrome. She notes the weak literature behind corticosteroid premedication is based on older high-osmolality agents that modern imaging has left behind.

Dr. Amit Katyan, a radiologist at the Ottawa Hospital and its lead for CT, has built a protocol that works back through the history for signs the reaction affected airway, breathing or circulation, or required epinephrine, an emergency visit or admission. For mild and moderate reactions his team skips steroids and switches to a contrast agent the patient has not had before, an approach he says the guidelines favour.

For physicians, the first move is to reverse-engineer the label rather than act on it, since many notes describe an expected physiological reaction rather than a true allergy. A necessary contrast study should not be delayed for steroid premedication when the history points to a mild reaction and another  agent can be substituted.

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