What are the steps for managing hypersensitivity reactions?

Management of Hypersensitivity Reactions

For acute hypersensitivity reactions, immediately discontinue the offending agent and administer intramuscular epinephrine 0.3-0.5 mg (0.01 mg/kg in children <30 kg) into the anterolateral thigh for any signs of anaphylaxis, repeating every 5-10 minutes as needed, while mild cutaneous-only reactions can be managed with oral antihistamines and observation. 1, 2

Acute Management Algorithm

Immediate Recognition and Treatment

Severity determines treatment intensity:

• Anaphylaxis (respiratory symptoms, hypotension, multi-organ involvement): Administer intramuscular epinephrine immediately into the anterolateral thigh using a needle at least 1/2 to 5/8 inch long to ensure intramuscular delivery 2. Do not delay epinephrine administration—antihistamines alone are insufficient 1.

• Mild reactions (isolated urticaria, pruritus, limited angioedema without respiratory/cardiovascular symptoms): Administer H1 antihistamines such as diphenhydramine 50 mg orally 1. H2 antihistamines like ranitidine can be added for enhanced effect 1.

Critical Monitoring Requirements

All patients require continuous observation even after initial treatment:

• Monitor vital signs regularly and watch for progression to respiratory symptoms, hypotension, or gastrointestinal involvement 1
• Maintain observation for at least 1-2 hours after complete symptom resolution 1
• If any progression occurs, immediately administer intramuscular epinephrine regardless of initial mild presentation 1
• Obtain serum tryptase levels 30-120 minutes after severe reactions for diagnostic confirmation 3

What NOT to Do in Acute Management

Avoid these common pitfalls:

• Do not administer corticosteroids for mild cutaneous-only reactions—they have not been shown to prevent biphasic reactions in mild cases and may actually increase biphasic reaction risk in children 1
• Do not administer repeated epinephrine injections at the same site, as vasoconstriction may cause tissue necrosis 2
• Do not use epinephrine if the solution is colored, cloudy, or contains particulate matter 2

Prevention of Future Reactions

Documentation Requirements

Thorough documentation is essential for future management:

• Document the exact inciting agent (specific drug name, manufacturer, lot number if available) in the electronic health record allergy field 4
• Record all symptoms with timing of onset and treatments administered 4
• Use standardized symptom descriptors and phenotypes from allergy/immunology literature 4

Risk Stratification for Future Exposures

Management differs dramatically based on reaction severity:

For Mild Prior Reactions (isolated cutaneous symptoms)

• Premedication is NOT recommended—this represents a major change from prior American College of Radiology recommendations 4, 5
• Switching to an alternative agent is recommended when the inciting agent is known and feasible 4
• Direct switching is more effective than premedication, with breakthrough reaction rates of 6-8% with switching versus 25-28% with same agent 4, 1

For Severe Prior Reactions (anaphylaxis, bronchospasm, hypotension, cardiovascular symptoms)

Follow this algorithmic approach:

1. First priority: Consider alternative imaging or diagnostic studies that avoid the same drug class entirely 4

2. If no acceptable alternative exists:

   • Premedication IS recommended with corticosteroids (prednisone 50 mg orally at 13,7, and 1 hour before procedure) 5
   • Switching the contrast agent is recommended when the inciting agent is known 4
   • Perform the procedure in a hospital setting with rapid response team capabilities, including personnel, equipment, and supplies to treat anaphylaxis 4, 5

3. Understand the limitations: The number needed to treat with premedication is approximately 69 patients to prevent one reaction of any severity and 569 patients to prevent one severe reaction 5

Premedication Risks vs Benefits

The evidence supporting premedication is of very low quality, and risks must be weighed:

• Direct risks: Transient hyperglycemia lasting up to 48 hours, anticholinergic and sedative effects requiring a driver, transient leukocytosis, mood changes, potential infection risk 5
• Indirect risks: Diagnostic delay from the 13-hour premedication protocol, prolonged hospital stay for inpatients 4
• Key evidence: Premedication alone was not effective at preventing breakthrough reactions when the same agent was used (26% with premedication vs 25% without), but direct switching reduced breakthrough reactions to 3-6% 4

Special Populations and Circumstances

These patients do NOT require premedication:

• Patients with prior chemotoxic or physiologic reactions (not immune-mediated) 4, 5
• Patients with isolated shellfish allergy 4, 5
• Patients with isolated iodine allergy including topical povidone-iodine 4, 5
• Patients with history of immediate reaction to gadolinium-based contrast media receiving iodinated contrast—no cross-reactivity exists 4

Advanced Diagnostic and Management Options

Consider these in select high-risk patients:

• Skin testing: May be helpful for patients with severe hypersensitivity reactions, especially those with reactions in the past 6 months requiring repeat administration 4. European guidelines recommend skin testing with the culprit agent and alternative agents to identify a tolerated option 4. However, skin testing is not routinely performed in the United States and access may be limited 4.

• Rapid drug desensitization: Reserved for situations where there are no ideal treatment alternatives and the drug is essential 4, 6. This procedure induces temporary drug tolerance through gradually incremental drug doses 4.

• Prescreening: NOT recommended for patients without a reaction history—empirical testing has extremely low sensitivity and minimal positive predictive value (both ~0%) 4

Common Pitfalls to Avoid

• Do not confuse mild reactions with severe reactions—only severe reactions warrant premedication for future exposures 1
• Do not rely on premedication alone—switching agents is more effective than premedication and should be prioritized 4
• Do not delay epinephrine in acute management—it is the first-line treatment for anaphylaxis, not antihistamines or corticosteroids 1, 2
• Do not assume cross-reactivity between different contrast classes—iodinated contrast and gadolinium-based agents have no chemical similarity 4