What labs are useful in diagnosing an allergic reaction?

Laboratory Tests for Diagnosing Allergic Reactions

The most useful laboratory tests for diagnosing allergic reactions include specific IgE blood tests and serum tryptase measurements, which should be obtained during or shortly after the reaction to confirm the diagnosis and identify the causative agent. 1

Primary Diagnostic Tests

Blood Tests

1. Specific IgE Testing

   • Can be taken during the reaction or shortly afterward during hospitalization 1
   • Uses allergen bound to a solid matrix (CAP system) with fluorescent detection 1
   • Particularly useful when:
     • Skin testing is contraindicated
     • Patient is on medications that interfere with skin testing
     • Patient has skin conditions like dermatographism or severe eczema 1
   • Available for common allergens including:
     • Antibiotics (amoxicilloyl, ampicilloyl, penicilloyl G, penicilloyl V, cefaclor)
     • Latex
     • Chlorhexidine
     • Bovine gelatin 1

2. Serum Tryptase

   • Critical timing: Obtain within 1-3 hours after reaction onset
   • Elevated levels strongly suggest mast cell degranulation
   • Useful to differentiate anaphylaxis from mimics
   • Can help rule out systemic mastocytosis (by comparing baseline to reaction levels) 1

Skin Testing

While not a laboratory test per se, skin testing is a crucial diagnostic tool:

• Skin prick testing: >80% sensitivity and specificity 1
• Intradermal testing: Used when skin prick tests are negative but suspicion remains high 1
• Most useful for:
  • Latex
  • Beta-lactam antibiotics
  • Neuromuscular blocking agents
  • Induction agents
  • Protamine
  • Chlorhexidine 1

Test Selection Algorithm

1. Acute Phase Testing (during or immediately after reaction):

   • Serum tryptase (within 1-3 hours of reaction)
   • Specific IgE blood tests for suspected allergens

2. Follow-up Testing (after recovery):

   • Repeat specific IgE testing if initial results were negative
   • Refer for skin testing once antihistamines are discontinued

3. Special Considerations:

   • For suspected systemic mastocytosis: Compare baseline tryptase to reaction levels
   • For drug reactions: Test for specific antibodies against suspected medications

Tests to Avoid

• IgG or Total IgE Testing: Not recommended as they do not provide helpful information for allergic rhinitis management 1
• Patch Tests: Useful for contact allergic reactions but not helpful for anaphylactic reactions 1
• Tests for NSAIDs, Dextrans, or Iodinated Contrast Media: Skin tests not useful as reactions are typically non-IgE-mediated 1

Common Pitfalls and Limitations

1. False Negatives:

   • IgE antibodies may be consumed during severe reactions, leading to falsely negative results 1
   • Repeat testing may be necessary if initial results are negative but suspicion remains high

2. False Positives:

   • Positive specific IgE tests indicate sensitization but not necessarily clinical allergy 1
   • Positive predictive value of skin tests is less than 50% 1

3. Timing Issues:

   • Tryptase levels must be obtained within hours of the reaction
   • Antihistamines must be discontinued before skin testing 1

4. Test Interpretation:

   • Laboratory results must always be correlated with clinical history 1
   • A positive test alone does not establish clinical food allergy 1

By following this systematic approach to laboratory testing for allergic reactions, clinicians can more accurately diagnose the condition, identify causative agents, and guide appropriate management to reduce morbidity and mortality associated with allergic reactions.