May be of use when diagnostic uncertainty

Peaks at 60–120 min after onset of symptoms

Take samples at onset of symptoms then 2–4h and >24h (baseline) thereafter; label sample time clearly on request form

Two-fold increase (even within the reference range) indicative of anaphylaxis

Poor negative predictive value – rise often absent in food anaphylaxis

Baseline elevated >20μgm/L may be due to mast cell disorders including mastocytosis