Let’s begin with a quiz question:
Patients may be allergic to:
D. none of the above
If you answered anything but ‘D’, better keep reading.
Consider this scenario: if a patient is allergic to penicillin, you would document “penicillin” in the medical records. It would never occur to you to list “antibiotics” as an allergy, and you certainly would not call it a “carbon” allergy for slang, just because penicillin contains carbon.
This not an exercise in semantics, but a patient care issue. Iodine is a mineral we all need, intentionally added to our salt supply, so that people do not get iodine-deficiency goiters as they once did. (We could boost everyone’s protein intake by adding peanut powder to the salt supply, but people might understandably object.) So while some people may be allergic to compounds that happen to contain iodine, the iodine in the molecule is not the culprit.
Iodine-based contrast agents are widely used for CT and other x-ray studies. They light up blood vessels and enhance perfusing tissue. These agents are essential for diagnosing everything from clots, to tumor, to bleeding. Unfortunately, many patients do not get contrast studies they may benefit from, due to unnecessary confusion about allergies.
The most important step to avoiding confusion is to start calling contrast agents by their names, as you would do for any other drug, and to remove “iodine” from your allergy vocabulary. Like antibiotics, there is more than one kind of contrast agent. Radiologists routinely dictate the name of the agent used in their study reports, so if a reaction occurs, the information is readily available. The problem comes when we do not distinguish one agent from another. Most patients with a prior reaction were never told the name of the drug they received, and they incorrectly assume that an allergy to one means allergy to all. Not so, just like with other classes of medications.
Here’s a brief summary of the pertinent points regarding contrast allergy:
- Older ionic iodinated contrast agents were more allergenic, and are no longer commonly used most places. When there may be a history of a contrast allergy, always first try to determine the actual drug name. If unattainable, and the reaction occurred before circa 1990, it was almost certainly an older ionic contrast agent.
- If symptoms consisted of nausea, vomiting, or a brief warm sensation, it may not have been pleasant for the patient, but those are considered physiologic effects of IV contrast, not actual allergic reactions.
- If the reaction to an older ionic contrast agent was mild, such as hives with no trouble breathing, a newer nonionic agent (those commonly used today) will usually be tolerated. Oral steroid premedication may also be prescribed.
- If the former allergic reaction was severe, including anaphylaxis, using any iodinated contrast agent would be discouraged, even the newer less allergenic agents.
Other points to remember:
- Gadolinium based agents for MRI are completely different than iodinated contrast agents for CT. Allergies to gadolinium agents may occur, but more rarely.
- Topical iodine soap sensitivity is unrelated to IV contrast allergy. If a patient is allergic to a topical soap, list the soap allergy by name, to prevent confusion with IV contrast.
- The same goes for shellfish allergies, a reaction to the tropomyosin of the muscle protein. Like any other food allergy, this is not associated with IV contrast.
Having worked in radiology for over 25 years, I cannot count the number of patients who have “iodine allergy” [sic] listed in their records, when they have never even received one of the current class of contrast agents. Once an allergy is posted in a medical record, people are reticent to remove it — but this is precisely what should be done whenever information is inaccurate. A few questions in the history taking, and confirmation of agents used for prior injections, are usually all that is required to clear a patient for a study they may need. Better to clarify now, than to wait until there is an immediate situation with less time to act, like a possible pulmonary embolism, stroke, or trauma. And for anyone in the electronic medical records business, it would be best if “iodine” were not allowed as an allergy choice option. It is not only nonsensical; it may prevent someone from obtaining a prompt diagnosis when they need it most.
Dr. Cullen Ruff is in private practice, and an associate professor of radiology at Virginia Commonwealth University in Fairfax, VA. He has no disclosures or conflicts. More information about contrast agents can be found here.