Laboratory Testing in Anaphylaxis
The primary laboratory test to order in the ED for anaphylactic reactions is serum mast cell tryptase, drawn in a specific three-sample protocol: immediately during resuscitation, at 1-2 hours post-symptom onset, and at 24 hours or during convalescence. 1
Essential Tryptase Sampling Protocol
The three-sample tryptase protocol is critical for accurate diagnosis:
- First sample: Draw 5-10 mL of clotted blood as soon as feasible after resuscitation has started—do not delay resuscitation to obtain this sample 1
- Second sample: Obtain at 1-2 hours after the start of symptoms 1
- Third sample: Collect at 24 hours or during convalescence (such as in a follow-up allergy clinic) to establish baseline tryptase levels, as some individuals have naturally elevated baseline levels 1
- Label all samples with the exact time and date of collection 1
Why This Three-Sample Approach Matters
The timing is algorithmically structured because tryptase levels peak at different times depending on the severity and mechanism of the reaction. The baseline sample is essential because approximately 5-7% of the population has elevated baseline tryptase levels unrelated to anaphylaxis, which would lead to false-positive interpretation if you only drew acute samples. 2
Additional Laboratory Considerations
Allergen-specific IgE testing can be drawn during the acute reaction or shortly afterward, but should be repeated 4-6 weeks later if initially negative, as IgE antibodies may be temporarily consumed during the acute reaction. 2
Critical Pitfall: IgE Testing Limitations
Approximately 23.6% of allergic reactions are non-IgE-mediated and will have negative blood tests despite true clinical allergy. 2 This means:
- Never rule out anaphylaxis based on negative IgE testing alone 2
- The presence of drug-specific IgE indicates sensitization but is not proof that the drug caused the reaction 2
- Sensitization does not equal clinical allergy—many patients with positive IgE tests tolerate the allergen without symptoms 2
Routine Laboratory Tests
While tryptase is the diagnostic test specific to anaphylaxis, standard supportive laboratory work may be indicated based on clinical presentation:
- Arterial blood gas determination should guide oxygen therapy where hypoxemia is a concern 1
- Continuous pulse oximetry is essential for monitoring respiratory status 1
- Hemodynamic monitoring is essential when administering intravenous epinephrine 1
What NOT to Do
Do not delay treatment to obtain laboratory confirmation. 1 The diagnosis of anaphylaxis is clinical, and epinephrine administration should never be postponed while waiting for tryptase results. Laboratory testing serves to confirm the diagnosis retrospectively and guide long-term management, not to determine whether to treat acutely. 3
Documentation Requirements for Laboratory Liaison
Coordinate with the hospital laboratory regarding proper handling of tryptase samples, as specialized processing may be required. 1 Ensure that the laboratory is aware of the clinical suspicion of anaphylaxis so samples are processed appropriately and results are expedited.