Intravenous (IV) dye is contrast dye given through the vein. It is also known as radiocontrast media (RCM). It is used widely in the United States for radiological studies such as angiograms, X-rays, magnetic resonance imaging (MRI), and computed tomography (CT) scans.
Adverse reactions to RCM are fairly common, though allergies are rarer. Allergic reactions to RCM are not truly allergic in nature, meaning that there is no allergic antibody present that causes the reaction. Rather, RCM acts to directly release histamine and other chemicals from mast cells.
The two major types of contrast used are iodinated contrast (used in most CT scans) and gadolinium-based contrast (used in most MRI scans that use contrast dye). The two types are quite different and are not thought to cross-react.
Within the iodinated contrast type, there are two major subtypes:
LOCM has become the preferred form of IV dye in recent years, given its better safety record. However, it's more expensive than HOCM.
These are reactions that may occur when you've been administered RCM:
The likelihood of a reaction to LOCM is much lower than that with HOCM, and the likelihood of a reaction to a gadolinium-based contrast (as used in MRIs) is even lower.
These factors appear to put people at higher risk for reactions to RCM:
Despite the popular myth, having a seafood and shellfish allergy does not place you at an increased risk of having a reaction to RCM. Shellfish allergy is due to the protein content of these foods, not the iodine content.
In addition, if you have an allergy to topical iodine cleaners or iodides, you're at no increased risk for reactions to RCM.
Unfortunately, there is no test available to diagnose an allergy to RCM. Skin testing and radioallergosorbent testing (RAST) have not been shown to be helpful in the diagnosis.
Small test doses are often not helpful, with reports of severe, life-threatening reactions occurring after small amounts of RCM given, as well as severe reactions with larger doses of RCM occurring after a person tolerates a small dose of IV dye.
An IV-dye allergy can only be diagnosed after symptoms have occurred. Otherwise, it's only possible to determine that a person is at increased risk of a reaction.
The treatment of an acute reaction to RCM is similar to that of an adverse reaction from any cause. Treatment may include injectable epinephrine and antihistamines, as well as the use of intravenous fluids for low blood pressure and shock.
It is common in patients with a history of non-severe pseudo-allergic reactions to RCM to be treated with a combination of oral corticosteroids, such as prednisone, and antihistamines, such as diphenhydramine (Benadryl) before any future contrast administrations.
Patients with a history of severe reactions should avoid radiocontrast media except in specific, severe circumstances under the direction of a physician.
If you're concerned about a potential reaction to RCM, talk to your doctor about the risks and benefits of performing a test with RCM and whether alternatives are available.
Your physician may be able to gain similar information by performing an MRI scan (which uses gadolinium-based contrast) rather than a CT scan. If a CT scan is required, ask whether LOCM rather than HOCM could be used.
In patients with a history of severe reactions, radiocontrast should be completely avoided except in specific extreme circumstances under physician supervision.