One of the most common myths I hear about allergies is that if you are allergic to shellfish, you cannot receive contrast with iodine. Contrast is used during certain medical tests like CAT scans. The contrast helps to get a better view of certain parts of the body during these tests.
In the past, it was thought that people who were allergic to shellfish were allergic to the iodine in the shellfish, and therefore they must also be allergic to contrast that contains iodine. However, research has shown definitively that allergy to shellfish is due to specific proteins in the fish and not due to iodine. There are many other foods that contain iodine as well. Iodine also naturally occurs in the body and is necessary for normal thyroid function.
Usually, people who have suspected reactions to iodine are actually allergic to other ingredients that may be mixed with the iodine. Recent studies have shown that people who have shellfish allergy have a similar risk of having an allergic reaction to contrast as the general population.
Let’s consider this patient:
A 45-year-old woman was seen in the emergency department after a car accident where she had some minor head trauma and needed to get a CAT scan of her head. Just before the CAT scan, she received contrast, and shortly thereafter she began to have difficulty breathing and began itching all over her body. She was treated for a suspected allergic reaction to the contrast. She had not had any previous reactions to contrast and did not have any history of food allergy. Before leaving the emergency department, she was told that since she is now allergic to iodine, she would need to also avoid shellfish. She was confused because she had just eaten shrimp a few days ago without any issues.
So is the patient suddenly now allergic to shellfish?
Likely not. Although it is possible to become allergic to foods that one has previously eaten without any issues, it is less likely to be the case here. As described above, there is no relation between the iodine in shellfish and allergy to contrast. So she could probably safely continue eating shellfish. To be sure, she can request to have allergy testing completed first.
An additional question to consider is: What should she do if she ever needs contrast again? Fortunately, there are preventative treatments that can be taken before any procedure requiring contrast to reduce the risk of having another reaction that she can discuss with her doctors.
Allergies Demystified 