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.

Types of Radiocontrast Media

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:

• Ionic high-osmolality contrast media (HOCM)
• Non-ionic low-osmolality contrast media (LOCM)

LOCM has become the preferred form of IV dye in recent years, given its better safety record. However, it's more expensive than HOCM.

Types of Reactions

These are reactions that may occur when you've been administered RCM:

• Mild reactions: These are relatively common, occurring in 3% to 15% of people receiving them. Most of these reactions are mild and include a feeling of warmth, nausea, and vomiting. Generally, these symptoms occur only for a short period of time and don't require treatment.
• Moderate reactions: These include severe vomiting, hives, and swelling, and occur in an estimated 0.02% to 2% of people receiving RCM. They frequently require treatment.
• Severe, life-threatening reactions: This includes anaphylaxis, and these reactions occur in 0.04% to 0.02% of people receiving RCM, with a death rate of one person in every 170,000.

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.

Risk Factors

These factors appear to put people at higher risk for reactions to RCM:

• Past reactions to RCM
• Asthma
• History of allergies
• History of heart disease
• History of kidney disease
• Taking beta-blockers
• Being elderly (at increased risk for severe reactions)

Diagnosis

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.

Treatment

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.

A Word From Verywell

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.