Should Antihistamines Be Stopped Before Allergy Testing?
Yes, H1 antihistamines must be discontinued before skin prick or intradermal allergy testing because they suppress the histamine-mediated wheal and flare response that is essential for accurate test interpretation.
Discontinuation Requirements
H1 Antihistamines (First Priority to Stop)
Drugs with antihistamine activity must be discontinued a few days before testing 1. The specific withdrawal period depends on the medication's half-life:
- Desloratadine: Stop 6 days before testing (longest elimination half-life of 27 hours) 2
- Second-generation H1 antihistamines (cetirizine, fexofenadine, loratadine, levocetirizine): Stop at least 3 days before testing 3
- First-generation H1 antihistamines (hydroxyzine, chlorphenamine/chlorpheniramine, diphenhydramine): Stop 3-7 days before testing due to more potent suppression 4
Other Medications That Interfere
The evidence shows these medications also suppress skin test responses and should be discontinued if clinically feasible 3:
- Tricyclic antidepressants (TCAs): Odds ratio 6.33 for negative histamine test - highest interference after H1-blockers
- Benzodiazepines: Odds ratio 5.01 for negative histamine test
- Atypical antidepressants (quetiapine, mirtazapine): Odds ratio 3.11 for negative histamine test
- H2-antagonists: Odds ratio 2.91 for negative histamine test (minimal but measurable interference)
Medications That Do NOT Need to Be Stopped
Important clinical distinction: The following medications show no significant interference with skin testing and can be continued 3:
- SSRIs (selective serotonin reuptake inhibitors): 97% had positive histamine controls
- SNRIs (selective norepinephrine reuptake inhibitors): 100% had positive histamine controls
- Proton pump inhibitors (PPIs): 97% had positive histamine controls
- Oral or inhaled corticosteroids: No need to discontinue 1
Clinical Rationale
The mechanism is straightforward: skin testing relies on allergen-induced mast cell degranulation releasing histamine, which creates the visible wheal and erythema response. H1 antihistamines competitively block histamine receptors, preventing this visible reaction even when IgE-mediated sensitization exists 5.
First-generation antihistamines are more problematic than second-generation agents. Hydroxyzine suppressed histamine-induced skin reactions in >50% of patients, while second-generation antihistamines only prevented positive reactions in 10-20% of individuals 4. This explains why longer discontinuation periods may be needed for first-generation agents.
Verification of Adequate Withdrawal
Always use positive and negative controls 1, 5:
- Positive control: Histamine solution must produce a wheal ≥3 mm for prick testing or ≥5 mm for intradermal testing
- Negative control: Physiological saline must remain negative
If the histamine control is negative or inadequate, the test results are invalid and testing should be rescheduled after ensuring complete medication washout 5.
Common Pitfalls to Avoid
Don't assume all antidepressants interfere equally: Only TCAs and certain atypical agents (mirtazapine, quetiapine) significantly suppress responses; SSRIs and SNRIs do not 3
Document all medications taken within the past week on the testing record 5 - this is essential for interpreting equivocal results
Don't stop corticosteroids unnecessarily: Unlike antihistamines, systemic or inhaled corticosteroids do not interfere with immediate hypersensitivity skin testing 1
Consider clinical context for H2-antagonists: While they show minimal interference (odds ratio 2.91), the effect is less pronounced than H1-blockers. If the patient requires H2-antagonists for medical reasons, testing can proceed with awareness of potential minor suppression 3
Timing after anaphylaxis: After systemic anaphylactic reactions, there may be a refractory period. Testing should not be performed within the first week after such reactions 6
Special Populations
Patients who cannot safely discontinue antihistamines (e.g., severe chronic urticaria requiring continuous therapy): Consider serum-specific IgE testing as an alternative, though it is less sensitive than skin testing 5, 7. The clinician should list all medications the patient has taken and note that results may be falsely negative 5.