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Key Features

Essentials of Diagnosis

  • Anaphylaxis is an acute life-threatening clinical syndrome that occurs when large quantities of inflammatory mediators are rapidly released from mast cells and basophils after exposure to an allergen in a previously sensitized patient

  • Anaphylactoid reactions

    • Mimic anaphylaxis but are not mediated by IgE antibodies

    • May be mediated by anaphylatoxins such as C3a or C5a or through nonimmune mast cell degranulating agents

  • Idiopathic anaphylaxis by definition has no recognized external cause

General Considerations

  • Common causes of anaphylaxis or anaphylactoid reactions

    • Drugs

      • Antibiotics

      • Anesthetic agents

    • Foods

      • Peanuts, tree nuts, shellfish, and others

    • Biologicals

      • Latex

      • Insulin

      • Allergen extracts

      • Antisera

      • Blood products

      • Enzymes

    • Monoclonal antibodies (eg, omalizumab)

    • Insect venoms

    • Radiocontrast media

    • Aspirin and other nonsteroidal anti-inflammatory drugs

    • Anesthetic agents

    • Idiopathic

Clinical Findings

Symptoms and Signs

  • Onset typically occurs within minutes after exposure to the offending agent and can be short-lived, protracted, or biphasic, with recurrence after several hours despite treatment

  • Anaphylaxis is highly likely when any one of the following three criteria is fulfilled:

    • Acute onset of an illness (minutes to several hours) with involvement of the skin, mucosal tissue, or both (eg, generalized hives, pruritus or flushing, swollen lips-tongue-uvula) and at least one of the following:

      • Respiratory compromise (eg, dyspnea, wheeze, bronchospasm, stridor, reduced peak expiratory flow, hypoxemia)

      • Reduced blood pressure or associated symptoms of end-organ dysfunction (eg, hypotonia [collapse], syncope, incontinence)

    • Two or more of the following that occur rapidly after exposure to a likely allergen for that patient (minutes to several hours):

      • Involvement of the skin-mucosal tissue (eg, generalized urticaria, itch-flush, swollen lips-tongue-uvula)

      • Respiratory compromise (eg, dyspnea, wheeze, bronchospasm, stridor, reduced PEFR, hypoxemia)

      • Reduced blood pressure or associated symptoms (eg, hypotonia [collapse], syncope, incontinence)

      • Persistent gastrointestinal symptoms (eg, crampy abdominal pain, vomiting)

    • Reduced blood pressure after exposure to a known allergen for that patient (minutes to several hours):

      • Infants and children: low systolic blood pressure (age specific) or greater than 30% decrease in systolic pressure

      • Low systolic blood pressure in children, defined as less than 70 mm Hg in those aged from 1 month to 1 year, less than (70 mm Hg + [2 × age]) in those 1–10 years of age, and less than 90 mm Hg in those 11–17 years

Differential Diagnosis

  • Other causes of shock along with cardiac arrhythmias

  • Respiratory failure associated with asthma

  • Mastocytosis

  • Hereditary angioedema

  • Scombroid poisoning

  • Vasovagal reactions

  • Vocal cord dysfunction

  • Anxiety attacks


  • Clinical history

Laboratory Findings

  • An absence of laboratory findings does not rule out anaphylaxis

  • Tryptase released by mast cells can be measured in the serum within 3 hours of onset of the reaction and may be helpful when the diagnosis of anaphylaxis is in question

  • However, tryptase ...

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