(This was written in response to Dr. Ehrlich’s post, “The Relevant Allergist,” which mentioned the persistent myth that shellfish-allergic patients will react to the iodine in dye used during x-rays, by a board-certified radiologist who practices in upstate New York.)
Radiologists years ago considered a patient’s history of an allergic reaction to shellfish as a contraindication to giving contrast for an IVP (kidney study), angiogram, CT (a.k.a. cat scan), or other radiologic studies. I say “years ago” because I have been in practice for 30 years and it was dogma when I started.
Because lots of radiology is about science, later studies were done that showed no connection between a patient’s shrimp “allergy” and the incidence of contrast reactions. The American College of Radiology produces a 98-page contrast manual. Its latest iteration Version 6 was issued in 1998. If you search it for the word “shellfish” you find only one mention on page 13:
The predictive value of specific allergies, such as those to shellfish or dairy products, previously thought to be helpful, is now recognized to be unreliable.
We don’t really understand contrast allergies or if they are even allergies at all. My patients who have broken out in hives with an injection may have no hives the next time. There is no definite connection between the hives and the anaphylactic reactions. Generally patients with lots of allergies are more likely to have reactions to contrast. If the benefit of the injection is very great, I go ahead and inject those with a history of hives. If the patient says his tongue swelled up and he can’t remember what happened before he woke up in the ER, I don’t inject unless the patient has been pre-treated with whatever combination of steroids and antihistamines is in vogue. If the patient has had a really bad contrast reaction, any future study requiring an injection ought to be done not in an outpatient office but in a real hospital with an emergency room.
All this being said, I have seen only 4 serious contrast reactions in my 30 years. They all did well. One did indeed have a shellfish history. Her reaction was not to injected “dye” but to the mixture of water and 2 tablespoons of water soluble contrast given by mouth. Because of the shrimp history we had no intention of injecting her. It did not protect her from the profound diarrhea she experienced.