How long after taking antihistamines (anti-histamine medications) can an allergy panel be performed?

Discontinuation of Antihistamines Before Allergy Panel Testing

Antihistamines should be discontinued 3-10 days before allergy panel testing, with specific timeframes varying by medication: diphenhydramine and fexofenadine for 3 days, loratadine for 7 days, cetirizine for 5-7 days, and hydroxyzine for 7-10 days. 1

Specific Discontinuation Timeframes by Medication Type

First-Generation Antihistamines

• Diphenhydramine (Benadryl): 3 days
• Hydroxyzine (Atarax, Vistaril): 7-10 days
• Chlorpheniramine: 3 days

Second-Generation Antihistamines

• Cetirizine (Zyrtec): 5-7 days
• Loratadine (Claritin): 7 days
• Fexofenadine (Allegra): 3 days

Rationale for Discontinuation

Antihistamines block histamine receptors, which directly interferes with allergy testing results by:

• Preventing the histamine-mediated wheal and flare response that is essential for interpreting skin tests
• Potentially causing false-negative results that could lead to missed diagnoses 1

This applies to both skin prick tests and intradermal tests, which are commonly used in allergy panels.

Clinical Considerations

Impact of Different Antihistamine Classes

• Second-generation antihistamines (cetirizine, loratadine, fexofenadine) have varying durations of action, requiring different washout periods 2
• First-generation antihistamines (diphenhydramine, hydroxyzine) generally have shorter half-lives but may require longer discontinuation periods due to their higher receptor affinity 1

Other Medications That May Interfere With Testing

While antihistamines are the primary concern, be aware of other medications that may affect allergy testing:

• H2 antagonists: 12 hours
• Leukotriene antagonists: 24 hours
• Tricyclic antidepressants: May require longer washout (up to 7 days) 2

Special Situations

Emergency Testing

If allergy testing must be performed urgently without adequate antihistamine washout:

• Include a histamine control to verify skin reactivity
• Be prepared for potential false-negative results
• Consider serum-specific IgE testing as an alternative, as it's not affected by antihistamine use

High-Risk Patients

For patients who cannot safely discontinue antihistamines (e.g., those with severe allergic conditions):

• Consider in vitro testing methods instead of skin testing
• If skin testing is essential, perform a histamine control test to assess for adequate skin reactivity

Common Pitfalls to Avoid

1. Inadequate washout period: Failing to discontinue antihistamines for the full recommended time can lead to false-negative results 1

2. Overlooking OTC medications: Many over-the-counter cold, sleep, and allergy medications contain antihistamines that patients may not recognize

3. Inconsistent guidance: Providing different discontinuation timeframes for the same medication creates confusion and may affect test reliability

4. Ignoring medication interactions: Some medications may prolong the effect of antihistamines, requiring longer washout periods

By following these specific discontinuation timeframes for antihistamines before allergy panel testing, clinicians can ensure more accurate test results and appropriate diagnosis and management of allergic conditions.