By Henry Ehrlich
Dr. Jen Gunter, who blogs about her field, ob/gyn, had a piece on the persistent myth of shellfish allergy and the iodine in radiocontrast dye used for x-rays. She writes:
From a nurse, “She can’t have providine prep, she has a shellfish allergy.”
From the patient, “I can’t have the contrast because I’m allergic to iodine and shellfish.”
The list of allergies placed in the chart by a doctor, “Radiocontrast. Iodine. Shellfish.”
It’s not the fault of the patients who are typically repeating the medical misinformation they have been given. However, the medical professionals who perpetuate the iodine-shellfish-radiocontrast allergy drive me batty because they either don’t understand the medicine involved or understand there is no link but don’t take the time to educate the patient and correct the chart. In short, I hate medical myths. Undoing what someone once said to somebody is hard because when someone has believed something for so long (whether they are a doctor, nurse, or patient) they are not always open to a change. I estimate that it takes a minimum of three conversations to under one incorrect piece of information.
Then Dr. Gunter dismantles the myths piece by piece.
We have repeatedly covered this subject. My favorite versions of it were by no coincidence both by doctors named Ehrlich. One was Dr. Paul Ehrlich’s letter to the New York Times that we linked to:
To the Editor:
I am an allergist and on staff at the N.Y.U. School of Medicine. Of all the cases referred to me by doctors in the hospital and from radiologists in particular, this supposed relationship between iodine sensitivity and radiologic studies and their avoidance has persisted in driving me totally nuts. I have colleagues whom I know and respect who persist in passing this myth along. Thanks for your piece. Now if we could only tattoo this some place obvious like on the foreheads of every chairman of every radiologic department.
Paul Ehrlich, M.D.
Another version of the medical reality was by a radiologist from upstate New York–my sister, Anne Ehrlich, MD.
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.
Dr. A
Thanks to Dr. Jen Gunter for her detailed treatment of this story. (Note: Dr. Gunter is also author of The Preemie Primer.