Objective: We aimed to determine the utility of serum mast cell tryptase to diagnose anaphylaxis.
Methods: As part of a venom immunotherapy trial, we performed 64 sting challenges. Blood samples were taken before the sting (baseline), and 15 min and 60 min after the sting. Tryptase was measured in baseline, 15 minute and 60 minute serum samples. Histamine was measured in baseline and 15 minute plasma samples. Eleven people had undisputed severe anaphylactic reactions; tryptase and histamine levels were assessed against this clinical gold standard diagnosis.
Results: Excluding mild reactions from the analysis, peak tryptase readings had sensitivity of 0.36 and specificity of 0.93 using the recommended cut-off range (< 12.0 µg/L). Receiver-operator curve analysis found a cut-off of 9.0 µg/L would improve diagnostic performance (sensitivity 0.55, specificity 0.93). Serial tryptase measurement was significantly more discriminatory; an increase in tryptase of 2.0 µg/L or greater had a sensitivity of 0.73 and specificity of 0.98. The addition of histamine measurements, defining a positive result by either a rise in tryptase or a rise in histamine, appeared to further increase sensitivity (0.90).
Conclusions: Clinicians should use caution when using serum tryptase to refute or support a diagnosis of anaphylaxis. Serial tryptase measurement increases sensitivity and specificity. Further studies using serial tryptase determinations in general emergency department practice, perhaps supplemented by histamine determinations, are warranted.