The diagnosis of anaphylaxis is by history and physical examination, and is easily made with a clear history of exposure, such as a bee sting, shortly followed by the multisystemic signs and symptoms described above. Unfortunately, diagnosis is not always so evident. Symptom onset is delayed over an hour in a small percentage of cases.3 Often, such as in food allergy, the inciting substance may not be known. The differential diagnosis of anaphylactic reactions is extensive, including vasovagal reactions, myocardial ischemia, arrhythmias, status asthmaticus, seizure, epiglottitis, hereditary angioedema, foreign-body airway obstruction, carcinoid, mastocytosis, and drug reactions.1 The most common anaphylaxis imitator is a vasovagal reaction, which is characterized by hypotension, pallor, bradycardia, diaphoresis, weakness, and sometimes syncope.3

Laboratory values have essentially no role in making the diagnosis. Histamine levels are elevated for 5 to 30 min postreaction and thus are usually in decline by presentation to the emergency department.3 Tryptase is a neutral protease of unknown function in anaphylaxis that is found only in mast-cell granules and is released with degranulation. Serum tryptase levels are elevated for several hours and are useful for research purposes for later confirmation of an anaphylactic episode. 1

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