Contrast agents: immediate and delayed reactions

No. Some immediate reactions are truly IgE-mediated. Others may involve non-IgE-mediated mast-cell activation. The older terms ‘anaphylactoid’ or ‘non-specific histamine release’ are used less often today.

A temporary warm sensation, an unusual taste, some nausea or a vasovagal episode may be physiological effects rather than allergy.

Non-immediate reactions appear several hours or days after injection. They often present as a maculopapular eruption, which may itch.

Most reactions are mild to moderate. Very rarely, a contrast agent may be involved in a severe cutaneous adverse reaction (SCAR).

Kidney function may need assessment before some examinations, but this is a different issue from allergy. For gadolinium-based agents, nephrogenic systemic fibrosis also concerns kidney safety, not an allergic mechanism.

A reaction to an iodinated contrast medium is a reaction to a contrast-agent molecule, not to ‘iodine’ in general. ‘Iodine allergy’ is not an appropriate diagnosis.

Betadine® / povidone-iodine

A reaction to an ICM does not automatically rule out povidone-iodine (Betadine®). Conversely, a reaction to Betadine does not predict a reaction to an ICM.

When contact dermatitis occurs with an iodinated antiseptic, it concerns the antiseptic product or its constituents, not a supposed iodine allergy.

Shrimp or fish allergy ≠ ICM allergy

Shellfish allergens are proteins, particularly tropomyosin, not iodine. Seafood allergy does not specifically predict a reaction to iodinated contrast.

Two chemically different families

No immunological cross-allergy between iodinated contrast media and gadolinium-based agents has been demonstrated. Any previous reaction to a contrast agent must nevertheless be reported.

Avoid both shortcuts: ‘reaction to iodinated contrast = gadolinium prohibited’ and ‘gadolinium = necessarily risk-free’.

Around the time of an examination, the patient may also be exposed to other substances or materials:

• an antiseptic, particularly chlorhexidine;
• equipment that may contain latex;
• adhesives;
• associated medicines;
• other devices.

An immediate reaction should therefore not automatically be attributed to the contrast agent. All exposures and their timing must be reconstructed, especially if contrast-agent testing is negative or the timing seems unusual.

• exact name of the ICM or GBCA; date and time; examination performed; time of injection;
• delay before the reaction; symptoms; treatment received; radiology department report;
• photographs of a delayed eruption, if possible.

For a moderate or severe immediate reaction, tryptase may be measured within 1 to 4 hours, followed by a baseline value after the reaction has resolved.

Do not take premedication on your own. Recent recommendations place greater emphasis on identifying the responsible product and choosing a suitable alternative than on viewing premedication as absolute protection.

Not necessarily. Cross-reactions occur between some ICMs and also between some gadolinium-based agents, but they are not universal. The assessment often aims to identify a tolerable alternative. After some severe drug eruptions, the strategy is much more restrictive and specialised.

Exact product · delay before reaction · symptoms · investigations performed · treatments received · any photographs · questions to ask