Is a shellfish allergy a concern for a patient undergoing a coronary computed tomography (CT) scan?

Shellfish Allergy is NOT a Concern for Patients Undergoing Coronary CT

A shellfish allergy is not a contraindication for patients undergoing coronary CT with iodinated contrast media, as this is a long-standing medical myth that has been debunked. 1

Understanding the Myth

The misconception that patients with shellfish allergies are at increased risk for reactions to iodinated contrast media likely originated from a 1975 survey that showed 15% of patients with contrast reactions reported shellfish allergies. However, similar proportions reported allergies to other foods like milk and eggs in the same survey 1. This led to the incorrect assumption that iodine was the common allergen.

Scientific Facts:

• Shellfish allergies are caused by proteins like tropomyosin, not iodine 1, 2
• Iodine is an essential nutrient that cannot be recognized as an antigen by the immune system 1
• The allergy risk from contrast media is not related to seafood or shellfish allergies 1

Current Guidelines for Coronary CT

The 2021 AHA/ACC/ASE/CHEST/SAEM/SCCT/SCMR guidelines for chest pain evaluation list several contraindications for CCTA, including:

• Allergy to iodinated contrast (not specifically shellfish)
• Inability to cooperate with scan acquisition
• Clinical instability
• Renal impairment
• Contraindication to beta-blockade when needed 1

Notably, shellfish allergy is not listed as a specific contraindication.

Risk Assessment for Contrast Reactions

The risk of anaphylactoid reactions to contrast media is approximately:

• 1% for general reactions
• 0.04% for severe reactions 1
• 0.0066% for iohexol-related events (in one large study) 1

These risks are not increased in patients with shellfish allergies compared to patients with other food allergies or asthma 3.

Management of Patients with Prior Contrast Reactions

For patients with a history of prior contrast reaction (not shellfish allergy):

• Consider premedication with 50mg prednisone 13 hours, 7 hours, and 1 hour before the procedure
• Add 50mg diphenhydramine 1 hour before the procedure 1

A common alternative regimen is:

• 60mg prednisone the night before and morning of procedure
• 50mg diphenhydramine 1 hour before procedure 1

Practical Approach for Patients with Shellfish Allergy

1. Do not exclude patients with shellfish allergies from coronary CT with contrast
2. Do not premedicate based solely on shellfish allergy history
3. Do assess for history of prior contrast reactions (which is relevant)
4. Do follow standard protocols for contrast administration

Common Pitfalls to Avoid

1. Unnecessary premedication - Pretreatment of patients with steroids based only on shellfish allergy history has potential adverse effects (e.g., hyperglycemia in diabetic patients) without demonstrated benefit 1

2. Propagating the myth - Asking about shellfish allergies specifically in pre-angiogram assessment perpetuates the misconception 4

3. Confusing shellfish allergy with contrast allergy - These are completely different hypersensitivity mechanisms with no cross-reactivity 1, 3

4. Delaying necessary imaging - Avoiding or delaying coronary CT due to shellfish allergy concerns may delay important diagnostic information

In summary, while appropriate screening for true contrast allergies remains important, shellfish allergies should not be a concern when considering coronary CT with iodinated contrast media.