Should I stop taking oral antihistamines (AH) and inhaled corticosteroids (ICS) before testing for Immunoglobulin E (IgE) and eosinophils?

Discontinuation of Oral Antihistamines and ICS Before IgE and Eosinophil Testing

Yes, you should stop taking oral antihistamines before IgE testing, but inhaled corticosteroids (ICS) can generally be continued.

Oral Antihistamines

• Oral antihistamines should be discontinued before allergy testing as they can suppress skin test responses and lead to false-negative results 1

• The recommended discontinuation period varies by medication:

  • Cetirizine: 5-7 days 1
  • Diphenhydramine: 3 days 1
  • Fexofenadine: 3 days 1
  • Hydroxyzine: 7-10 days 1
  • Loratadine: 7 days 1

• For serum IgE testing (blood tests), antihistamines do not need to be discontinued as they don't affect laboratory measurements of IgE antibodies 1

Inhaled Corticosteroids (ICS)

• Inhaled corticosteroids can generally be continued before allergy testing 1
• According to multiple guidelines, ICS are listed among "drugs that may be continued" during allergy testing 1
• ICS typically have minimal systemic effects and do not significantly interfere with skin test or serum IgE results 1

Effect on Eosinophil Testing

• Oral antihistamines have minimal direct effect on blood eosinophil counts 2
• However, systemic corticosteroids (oral or injectable) significantly decrease blood eosinophil counts by approximately 30-36% within one month 3
• After discontinuation of systemic corticosteroids, eosinophil counts may take several weeks to return to baseline levels 3
• ICS at standard doses have minimal effect on blood eosinophil counts compared to systemic corticosteroids 3

Important Considerations

• The negative predictive value of skin testing is more than 95%, while the positive predictive value is less than 50% - meaning false positives are more common than false negatives 1
• If you're having a food challenge test, more medications need to be discontinued, including H2 antagonists (12 hours before), leukotriene antagonists (24 hours before), and various other medications 1
• For methacholine challenge testing (used for asthma diagnosis), additional medication restrictions apply, including stopping short-acting bronchodilators for 8 hours and long-acting bronchodilators for 48 hours 1

Clinical Recommendation

• For skin testing: Discontinue oral antihistamines for the appropriate duration (3-10 days depending on the specific medication) 1
• For blood IgE testing: No need to discontinue antihistamines 1
• For eosinophil counts: Continue ICS, but be aware that recent use of systemic corticosteroids may suppress results for several weeks 3
• Always inform your allergist about all medications you are taking, as they may need to adjust the testing protocol accordingly 1

Common Pitfalls to Avoid

• Failing to stop antihistamines before skin testing, which can lead to false-negative results 1, 4
• Unnecessarily stopping ICS before testing, which could worsen asthma control without improving test accuracy 1
• Not accounting for recent systemic corticosteroid use when interpreting eosinophil counts 3
• Using standard medication panels for allergy testing rather than testing for specific suspected allergens 1