Not all Allergies are Anaphylaxis!

Anaphylaxis is defined as:

  • Severe life-threatening systemic hypersensitivity reaction
  • Where BOTH of the following criteria are met:
    1. Sudden onset & rapid progression
    2. Life-threatening compromise of ONE or MORE of: Airway/Breathing/Circulation

Supporting Signs:

  • Skin/Mucosal changes (flushing, urticaria, angioedema) often occur but are absent/subtle in 20% of cases.
  • Gastro-intestinal symptoms (vomiting, diarrhoea, abdominal pain) can be associated anaphylaxis.

Note: Skin/mucosal or GIT signs WITHOUT Airway/Breathing/Circulatory compromise DO NOT constitute anaphylaxis


Note: The recommendation to “lie patients flat”, may not be tolerated esp. if if the patient presents with airway or breathing compromise, and they should be allowed to sit in a comfortable position.



Look for precipitants and remove if possible.

Common precipitants include:

  • Food
  • Drugs
  • Stings
  • Idiopathic in up to 20%

Rapidity of onset from stimulus varies according to the stimulus type. As you would suspect injection/sting has a faster onset time compared to ingestion, however, there is significant overlap.



We should send mast cell tryptase, in suspected anaphylaxis at the following time points

  1. As soon as possible after emergency treatment has started
  2. Within 1–2 hours (but no later than 4 hours) from the onset of symptoms.

However, if the patient presents late we may only get sample 2


Observation (for Biphasic reaction):

The data on biphasic reactions is poor, and highly varied. Studies quote rates between 1-20% and onset times from 1hr – 72hrs. Biphasic reactions are thought less likely among people with food-induced anaphylaxis, but those presenting  with hypotension or  idiopathic anaphylaxis may be at increased risk.

NICE note the following:

  • 60% of biphasic reactions will occur within 8hrs of initial reaction.
  • Patients should be observed in hospital for 6-12hrs minimum.

Resus council suggest prolonged (24hr) observation in these groups:

  • Severe reactions with slow onset caused by idiopathic anaphylaxis.
  • Reactions in individuals with severe asthma or with a severe asthmatic component.
  • Reactions with the possibility of continuing absorption of allergen.
  • Patients with a previous history of biphasic reactions.
  • Patients presenting in the evening or at night, or those who may not be able to respond to any deterioration.
  • Patients in areas where access to emergency care is difficult.

If the patient is Discharging/Self-discharges from ED:

We need to ensure that patient is aware of the possibility of a biphasic reactions, and document safety netting given.

  • Use one adrenaline auto-injector and then call 999, ask for an ambulance, and state ‘anaphylaxis’, even if symptoms are improving.
  • Lie flat with the legs raised in order to maintain blood flow. If the person has breathing difficulties, they should sit up to make breathing easier.
  • Use the second auto-injector if they don’t start to feel better 5 to 15 minutes after the first injection.

Prescribing Adrenaline Injectors:

This will typically happen from the ward after an observation period. NICE recommend that patients with a documented anaphylaxis should receive the following:

  • a demonstration of the correct use of the adrenaline injector and when to use it
  • a prescription for 2 adrenaline injectors, with advice to carry the injectors with them at all times


Further Reading




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