Contrast media and seafood allergy link is fishy

An old medical myth — that patients who are allergic to seafood are at risk of adverse reactions to radiologic contrast media — persists even among cardiologists and inter-ventional radiologists, despite having been thoroughly debunked.

In a survey of 231 specialists at six academic medical centers, 69% of the physicians admitted asking patients about seafood allergy before radiologic procedures using contrast media or pre-medicating seafood-allergic patients with corticosteroids or antihistamines before the procedure.

Numerous studies over the past 30 years have failed to find any special relationship between seafood allergy and adverse reactions to radiologic contrast media (RCM). According to some small studies, atopic patients in general may have a four-fold to five-fold increased risk of adverse events in response to RCM. However, the baseline rate of these events is so low that even if these studies were to be confirmed in larger populations, less than 1% of atopic patients would be affected.

About 10 million procedures using RCM are conducted every year in the United States. Life-threatening reactions occur in about 0.2% of patients receiving high-osmolarity contrast media and 0.04% of patients receiving low-osmolarity contrast media.

The origin of the seafood-allergy myth is unknown. But St. Louis University, has traced it at least as far as a 1975 paper in the American Journal of Roengenology that stated that 15% of patients who experienced adverse reactions to RCM reported having seafood allergy. The authors of that study hypothesized that the iodine in seafood cross-reacted with the iodine in RCM. They never verified those patient reports, however, and similar percentages of patients with adverse reactions in their study reported allergies to other common foods such as milk and eggs.

Since then, it has been determined that seafood allergy is mediated by immunoglobulin E (IgE) antibodies to proteins in meat, with iodine playing no role. Furthermore, IgE does not mediate severe RCM reactions. The combination of these two findings effectively discounts the hypothesis of iodine cross-reactivity.

For this study, a doctor and a colleague mailed anonymous questionnaires to 231 faculty members at six prominent academic medical centers in the Midwest. Of the individuals queried, 49% responded.

The survey consisted of eight brief questions, but only two of them related to seafood allergy and RCM. The other six were intended as distractors. The first seafood-related question was, “Do you or someone on your behalf inquire about a history of seafood or shellfish allergy prior to administration of contrast media? For 65% of the radiologists and 89% of the cardiologists, the answer was “Yes”.

The second question was, “Would you withhold RCM administration or recommend pretreatment with corticosteroid and/or antihistamines based on a history of seafood or shellfish allergy? Overall, 35% of the radiologists and 50% of the cardiologists answered, ‘Yes’.

While 69% of the total respondents said that they would ask patients about seafood allergy, only 37% said that they would change management based on that information. That suggests that about 32% would ask the question even if the answer would not affect patient management.

Merely asking that question may serve to perpetuate the myth among patients. He pointed to a separate study indicating that 65% of patients with seafood allergy had either read or been told by their physician to avoid RCM, and 92% believed that iodine in seafood was responsible for their allergy.

Several physicians in the audience rose to describe their experiences with this medical myth. One described a radiologic technician who received an official reprimand for failing to ask a patient about seafood allergy. Another physician said that at his institution no allergic patients were allowed to receive RCM unless they were premedicated.

If radiologists want to pre-medicate every food allergic person, that’s fine. They don’t believe based on the percentages they found that it’s a necessary practice.

A third physician said that at his institution, the computer system automatically categorized every patient with a seafood allergy as being sensitive to RCM, and every patient who was sensitive to RCM as having a seafood allergy. That has not been changed, but patients who were seen before the change will have that erroneous information persist in their records until someone changes it manually.