Why Should You Avoid CT Scan Dye If You Have Shellfish Allergies?

When scheduling a cat scan, or CT, or other test requiring a contrast injection, experts may advise premedicating or using a different type of dye for patients with seafood allergies because those individuals experience a higher risk of having adverse reactions to the iodine in most contrast dyes. Most dyes contain iodine, a form of salt, and several other chemicals. People with seafood allergies develop reactions upon ingesting seafood because of immunoglobin E hypersensitivity; that same immune system response presents upon exposure to iodine-containing contrast dye.

According to the University of California, San Francisco, the likelihood of showing adverse reactions to iodine dyes varies widely among people with shellfish allergies. Generally, people with mild reactions, such as vomiting, diarrhea and skin reactions, tolerate all kinds of iodine dyes.

Most people with more severe reactions to shellfish, such as difficulty breathing, hives and shock, handle iodine-based dyes well. However, people with shellfish allergies have, on average, a 3 percent higher risk of developing allergies to iodine dyes than the general population. Although that means very few people ever react abnormally to dyes, treating physicians generally administer anti-allergy medicines, such as corticosteroids, or prescribe non-iodine dyes for all patients with shellfish allergies.

In addition to shellfish allergies, people with egg, milk or chocolate allergies have a 2.9 percent chance of reacting to contrast dyes, as do 2.6 percent of people with fruit and strawberry allergies, notes UCSF.