Most of these reactions occur within an hour of receiving contrast dye and many occur within the first five minutes. However, there can sometimes be delayed reactions that occur up to a week later.

The symptoms can include serious allergic-like symptoms, such as skin reactions or trouble breathing.

This article will explore the types of contrast dye (also called radiocontrast media, or RCM), risk factors for reactions, and how a contrast dye allergy is treated.

What Is IV Contrast?

IV contrast is a solution given through a vein that helps highlight and outline structures, such as organs, blood vessels, and tumors, to distinguish them from other tissues during imaging. This allows the radiologist or other doctors who read the scans to see the anatomy in detail.

IV contrast is commonly used with:

Computed tomography (CT) scans, which use a series of X-rays along with a computer to view structures inside the body. Magnetic resonance imaging (MRI), which uses a magnetic field and computer to create images Angiograms, which are X-rays that examine the path the contrast takes through blood vessels

Types of Contrast Dye

There are two major types, or classes, of IV contrast that are used in diagnostic imaging; iodinated contrast dye and gadolinium-based contrast dye.

Iodinated contrast dye contains iodine and is used in most CT scans and other types of imaging involving X-rays. The iodine helps to get visuals inside hollow spaces, such as blood vessels and organs.

There are two major subtypes in this class:

Non-ionic low-osmolality contrast media (LOCM): The iodine is bound to organic (non-ionic) compounds and is more diluted. Ionic high-osmolality contrast media (HOCM): The compounds can break into separate particles called ions and the iodine is more concentrated.

LOCM has become the preferred form of IV dye. However, it’s more expensive than HOCM.

Gadolinium-based contrast dye (GBCD) contains a rare earth metal that enhances MRI scans.

The two main types are quite different. Having a reaction to one doesn’t necessarily mean you will have a reaction to the other.

However, you should always discuss all previous reactions with your healthcare provider.

Types of Contrast Dye Reactions

The severity of a contrast dye reaction can range from mild to severe and life-threatening.

Contrast dye acts to directly release chemicals, such as histamine. This triggers allergic-like symptoms. A true allergy occurs due to the production of antibodies, which are specialized immune proteins. This typically does not happen with contrast dye reactions, but it has been described.

Mild Reactions

Mild reactions are relatively common, occurring in 3% to 15% of people receiving contrast.

Most of these reactions include:

A feeling of warmthNauseaVomiting

Generally, symptoms occur for a short period and don’t require treatment.

Moderate to Severe Reactions

Moderate reactions can include severe vomiting, skin reactions, and swelling, and occur in about 0.02% to 2% of people receiving contrast. They require treatment.

Severe reactions include anaphylaxis, a life-threatening emergency that can lead to difficulty breathing. Severe reactions occur in 0.04% to 0.02% of people receiving contrast, with a death rate of one person in every 170,000. Anaphylaxis is a type of allergic reaction.

Symptoms of a moderate or severe reaction that require urgent medical care include:

Severe vomiting Hives Difficulty breathing Swelling in the throat High pitched sound when breathing Convulsions Fast heart rate Cardiac arrest, which is a sudden loss of consciousness, breathing, and pulse

Risk Factors

These factors appear to put people at higher risk for adverse or allergic reactions to contrast dye:

Past reactions to a similar type of contrast Asthma Allergies Heart disease Kidney disease Taking beta-blockers, which are medications commonly used for a variety of conditions, including heart conditions

Older adults also have an increased risk for severe reactions.

Diagnosis

Unfortunately, there is no test available to diagnose a contrast dye allergy in advance. Skin testing and blood testing to look for allergies are often not helpful in the diagnosis.

In addition, if you have an allergy to topical iodine cleaners or iodides, you’re at no increased risk for these reactions.

Reactions are unpredictable and small test doses do not indicate whether or not a reaction will occur when a regular amount is given. There are even reports of severe, life-threatening reactions to contrast dye occurring after a person had tolerated a small test dose of IV dye.

Treatment

The treatment is symptomatic, meaning that treatment is given to alleviate the specific effects of the reaction.

Treatment may include the following:

Injectable epinephrine relaxes tubes of the lungs called bronchi, allowing easier breathing Antihistamines, drugs that block the actions of histamine IV fluids for treatment of low blood pressure and shock

Pre-Medication for Contrast Allergy

If you’ve had a non-severe reaction to contrast dye and need to use a similar type for more imaging, your medical provider will likely recommend pre-medication. This involves taking medication in advance of receiving contrast dye to lower the risk of a reaction.

It is common to be treated with a pre-medication combination of oral corticosteroids, such as prednisone, and antihistamines, such as Benadryl (diphenhydramine).

If you’ve had a history of severe reactions, your healthcare provider will avoid using the same class of contrast dye except in specific circumstances. Typically, the reaction is agent-specific and you might be able to use another iodinated contrast from a different class that is low in osmolarity and ionicity if needed.

Summary

IV contrast dye is a solution that’s put into the bloodstream during medical imaging in order to make internal structures, such as organs and blood vessels, easier to see.

The two main types of contrast are gadolinium-based contrast used for MRIs and iodinated contrast used for CT scans and other imaging with X-rays.

Having a reaction to iodinated contrast does not mean you will react to gadolinium-based contrast and vice versa.

Mild reactions to contrast dye are fairly common and don’t require treatment. In rare cases severe and life-threatening emergencies can occur.

A Word From Verywell

If you’re concerned about a potential reaction to contrast dye, talk to your healthcare provider about the risks and benefits of having a test with contrast and whether alternatives are available. If you have a history of reactions to contrast dye, always make your healthcare provider aware of any previous reactions.

If you’ve had a reaction to contrast dye used during a CT scan and you need imaging, your healthcare provider may be able to gain similar information with an MRI scan, which uses gadolinium-based instead of iodinated contrast.

These include:

Warm, flushed feeling through your body for a few secondsMetallic taste in your mouthItchinessFeeling like you’re urinating, but you’re not