Blood Work for Diagnosing Allergic Reactions in the Emergency Department

Timing of Tryptase Sampling

The primary blood test for diagnosing an allergic reaction in the ED is serum mast cell tryptase, which should be drawn as soon as feasible during resuscitation, followed by a second sample at 1-2 hours after symptom onset, and a third baseline sample at 24 hours or during convalescence, according to the Anaesthesia guideline society 1
The American College of Anaesthesia recommends drawing the first sample as soon as feasible after resuscitation has started, the second sample at 1-2 hours after the start of symptoms, and the third sample at 24 hours or during convalescence to establish baseline tryptase levels 1

The American Academy of Allergy, Asthma, and Immunology recommends that 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, with a strength of evidence not provided 2

Approximately 23.6% of allergic reactions are non-IgE-mediated and will have negative blood tests despite true clinical allergy, according to Praxis Medical Insights 2

The American Academy of Allergy, Asthma, and Immunology recommends that laboratory results must always be interpreted in clinical context, and never diagnose allergy based solely on positive specific IgE or tryptase without correlating with clinical history, with a strength of evidence not provided 2
The presence of drug-specific IgE indicates sensitization but is not proof that the drug caused the reaction, according to Praxis Medical Insights, with a strength of evidence not provided 2
Higher specific IgE levels correlate with increased probability of clinical reactivity, but the relationship varies by allergen, patient age, and clinical context, with a strength of evidence not provided 2
Sensitization does not equal clinical allergy, as many patients with positive IgE tests tolerate the allergen without symptoms, according to Praxis Medical Insights, with a strength of evidence not provided 2