Antihistamine Washout Period Before Allergy Testing
Discontinue antihistamines for a minimum of 3-7 days before skin prick or intradermal allergy testing, with the specific duration depending on the antihistamine's elimination half-life.
Drug-Specific Washout Periods
The washout period must be tailored to the specific antihistamine being used, as elimination half-lives vary significantly:
- Desloratadine requires 6 days due to its longest elimination half-life of 27 hours 1
- Most second-generation antihistamines (cetirizine, fexofenadine, loratadine, levocetirizine) require 3-5 days based on their pharmacokinetic profiles 1
- Ebastine requires 4 days, as research demonstrates the wheal and flare response returns to baseline by day 4 after discontinuation 2
- First-generation antihistamines (hydroxyzine, chlorphenamine) require 3-5 days despite shorter half-lives, as they demonstrate more potent suppression of skin test responses 1
Critical Testing Considerations
Drugs with antihistamine activity must be discontinued several days before testing to avoid false-negative results 1. The guideline specifically states that "drugs with anti-histamine activity must be discontinued a few days before testing" 1.
Key points for accurate testing:
- Antihistamines suppress both histamine and allergen-induced skin reactions, with first-generation agents like hydroxyzine showing >50% suppression of histamine responses even at standard doses 3
- Testing can proceed as soon as the patient has made full clinical recovery from any acute allergic reaction and antihistamine effects have worn off 1
- There is no need to discontinue oral or inhaled corticosteroids before skin testing 1
Alternative Testing Strategy
If antihistamines cannot be discontinued for medical reasons, proceed directly to serum-specific IgE blood testing instead of skin testing 4. The European Respiratory Society recommends this approach because:
- Blood testing is not affected by antihistamine use 4
- Sensitivity ranges from 0.58-0.90 and specificity from 0.56-0.65 compared to skin prick testing 4
- This is particularly useful for patients with extensive eczema, dermographism, or urticaria where skin testing is impractical 4
Common Pitfalls to Avoid
Do not perform skin testing while patients are taking antihistamines, as this will result in false-negative results and missed diagnoses 1. The clinician should document all medications taken within the past week, as many medications beyond antihistamines (including tricyclic antidepressants) may suppress skin test responses 1.
Never use antihistamine washout as a reason to delay urgent testing—simply switch to blood-based IgE testing when immediate results are needed 4.