Other laboratory markers that have been studied in anaphylaxis include serum histamine, urinary histamine metabolites, and concentration of angiotensin-converting enzyme (ACE). ACE is an enzyme involved in bradykinin catabolism, and angioedema is a known side effect for up to 0.7% of patients taking ACE-inhibiting medications.40 Regarding ACE levels, one study of peanut- and tree-nut-allergic individuals found an increased risk of severe pharyngeal edema in those patients having ACE concentrations in the lowest quartile (<37.0 mmol/L).28 Serum histamine is a less practical marker as it peaks within 10 minutes of anaphylaxis onset and returns to baseline values within 1 hour. Urinary histamine metabolites may be of better utility as these can remain elevated up to 24 hours following an episode of anaphylaxis; however, their utility in clinical practice is limited.