Will Benadryl (diphenhydramine) affect serum lab results for allergens, such as specific Immunoglobulin E (IgE) levels?

Effect of Benadryl on Serum Allergen Testing

Benadryl (diphenhydramine) should be discontinued at least 3-7 days before serum allergen-specific IgE testing, as it can affect the accuracy of test results, though the impact is significantly less than its effect on skin prick testing.

Mechanism of Impact on Allergen Testing

Antihistamines like Benadryl work by blocking histamine receptors, which primarily affects skin testing results rather than serum IgE measurements. However, there are important considerations:

• Serum IgE Testing: Measures circulating allergen-specific IgE antibodies in the blood

  • Less affected by antihistamines than skin tests
  • Detects allergic sensitization, not necessarily clinical allergy 1
  • Results must be correlated with clinical symptoms for proper diagnosis

• Medication Effects:

  • Antihistamines primarily affect in vivo tests (skin prick tests, intradermal tests) by blocking the histamine-mediated wheal and flare response 2
  • Serum testing can be especially useful when antihistamines cannot be discontinued 2

Clinical Implications

When to Use Serum IgE Testing

• When skin testing cannot be performed due to:
  • Patients on antihistamines that cannot be discontinued
  • Extensive dermatitis or dermatographism
  • Risk of anaphylaxis from skin testing 2, 1

Interpretation Considerations

• A positive specific IgE test indicates sensitization but not necessarily clinical allergy
• Correlation between serum IgE levels and clinical symptoms is essential 1
• Positive predictive value of IgE testing without clinical correlation is less than 50% 1
• Different laboratory assay systems may yield different results for the same sample 2

Factors Affecting Serum IgE Test Results

Several factors can influence serum IgE test results regardless of antihistamine use:

• Total IgE levels: Higher total IgE may affect the interpretation of allergen-specific IgE results 2, 3
• Patient age: IgE levels tend to decrease with age 3
• Gender differences: Males often have higher baseline IgE levels than females 4
• Type of IgE response: Affinity or epitope recognition patterns can affect test results 5

Best Practices for Testing

1. Medication Management:

   • If possible, discontinue Benadryl 3-7 days before testing
   • If antihistamines cannot be discontinued, serum testing is preferred over skin testing

2. Test Selection:

   • Serum allergen-specific IgE testing is useful when skin testing is contraindicated 2, 1
   • Different assay systems (ImmunoCAP, Turbo-MP, Immulite) may yield different results 2

3. Result Interpretation:

   • Always correlate test results with clinical history and symptoms 1
   • Consider that higher levels of specific IgE correlate better with symptomatic allergy 6
   • Cutoff values of approximately 10-12 kU/L may help distinguish symptomatic from asymptomatic allergy 6

Common Pitfalls to Avoid

• Relying solely on test results without clinical correlation
• Assuming that antihistamines have no effect on any allergy testing
• Failing to recognize that different laboratory methods may yield different results
• Overinterpreting low-positive results without considering clinical relevance

In summary, while Benadryl has less impact on serum allergen-specific IgE testing than on skin testing, discontinuing antihistamines before testing when possible provides the most reliable results for comprehensive allergy assessment.