Quick answer: anaphylaxis is a clinical diagnosis: sudden onset and rapid progression of life-threatening Airway, Breathing or Circulation problems, usually (but not always) with skin or mucosal changes. The first-line treatment is intramuscular adrenaline into the anterolateral thigh: 500 micrograms (0.5 mL of 1:1,000) for adults and children over 12, repeated after 5 minutes if there is no improvement. Antihistamines and steroids are not part of the initial emergency treatment.
Anaphylaxis is a favourite MRCEM topic because the Resuscitation Council UK (RCUK) guidance contains clear numbers that are easy to test. This guide covers recognition, adrenaline doses, refractory anaphylaxis, investigations and how long to observe.
Recognising anaphylaxis
RCUK describes anaphylaxis as likely when all three of the following are present:
- Sudden onset and rapid progression of symptoms.
- Life-threatening Airway and/or Breathing and/or Circulation problems, such as stridor, hoarse voice, wheeze, hypoxia, hypotension or collapse.
- Skin and/or mucosal changes, such as flushing, urticaria or angioedema.
Skin changes can be subtle or absent in up to 20% of cases, so their absence does not rule out anaphylaxis. Skin changes alone, without an ABC problem, are not anaphylaxis.
Immediate management
- Call for help and remove the trigger if possible, for example by stopping a drug infusion.
- Position the patient: lie flat, with the legs raised if hypotensive. Patients with breathing difficulty may prefer to sit up, and pregnant patients should lie on their left side. Do not let the patient stand or walk suddenly.
- Give IM adrenaline without delay.
- Give high-flow oxygen, establish monitoring and obtain IV access.
IM adrenaline doses
Adrenaline 1:1,000 (1 mg/mL) is given into the anterolateral aspect of the middle third of the thigh.
| Age | Dose | Volume of 1:1,000 |
|---|---|---|
| Adult and child over 12 years | 500 micrograms (300 micrograms if the child is small or prepubertal) | 0.5 mL |
| 6–12 years | 300 micrograms | 0.3 mL |
| 6 months – 6 years | 150 micrograms | 0.15 mL |
| Under 6 months | 100–150 micrograms | 0.1–0.15 mL |
Repeat the IM dose after 5 minutes if there is no improvement.
Fluids
If there is hypotension or shock, give a rapid IV crystalloid bolus:
- Adults: 500–1,000 mL. Large volumes may be needed in severe cases.
- Children: 10 mL/kg.
Refractory anaphylaxis
Refractory anaphylaxis is defined as anaphylaxis needing ongoing treatment, because of persisting respiratory or cardiovascular symptoms, despite two appropriate doses of IM adrenaline.
- Seek expert help early and involve critical care.
- Start a low-dose IV adrenaline infusion, following the RCUK refractory anaphylaxis algorithm.
- Continue IM adrenaline every 5 minutes until the infusion is running.
- Give further rapid fluid boluses.
IV bolus adrenaline is not recommended outside specialist settings, because of the risk of dangerous hypertension and arrhythmias. If cardiac arrest occurs, follow the standard ALS algorithm.
Antihistamines and steroids
- Antihistamines are not recommended as part of the initial emergency treatment. They do not treat the life-threatening features. A non-sedating oral antihistamine such as cetirizine can be given for skin symptoms once the patient is stable.
- Corticosteroids are no longer given routinely and must never be given in preference to adrenaline. They may be considered after initial resuscitation in refractory reactions or ongoing asthma or shock.
Mast cell tryptase
Tryptase helps to confirm the diagnosis afterwards. It must never delay treatment.
- Take a sample as soon as possible after emergency treatment has started.
- Take a second sample ideally within 1–2 hours, and no later than 4 hours, from the onset of symptoms.
- A baseline sample is taken at least 24 hours after the reaction has resolved, often at the allergy clinic.
How long to observe
Biphasic reactions can occur hours after the first reaction has settled. RCUK recommends a risk-stratified approach for adults and young people aged 16 and over:
| Observation from resolution of symptoms | When to consider it |
|---|---|
| 2 hours | Good response to a single dose of adrenaline given within 30 minutes of onset, complete resolution of symptoms, the patient already has unused auto-injectors and knows how to use them, and there is adequate supervision after discharge |
| At least 6 hours | Two doses of IM adrenaline were needed, or there is a previous biphasic reaction |
| At least 12 hours | Severe reaction needing more than two doses of adrenaline, severe asthma or severe respiratory compromise, possible ongoing absorption of the allergen (for example a slow-release medicine), late-night presentation, or difficulty accessing emergency care |
Children under 16 should be admitted under the paediatric team.
Before discharge
- Prescribe two adrenaline auto-injectors and train the patient to use them.
- Give advice about the risk of a biphasic reaction and about avoiding the trigger.
- Refer to a specialist allergy service.
Common exam traps
- Giving antihistamines or steroids first: adrenaline is the only first-line drug.
- Wrong route or strength: IM 1:1,000 into the thigh, not IV 1:10,000 and not subcutaneous.
- Waiting for a rash: skin signs are absent in up to 20% of cases.
- Sitting or standing a hypotensive patient up: this can precipitate cardiac arrest.
- Delaying treatment to take a tryptase sample: treat first.
- Discharging after 1 hour: the minimum observation period is 2 hours, and only for low-risk patients.
Summary
- Sudden onset ABC problem, with or without skin changes, is anaphylaxis until proven otherwise.
- IM adrenaline 500 micrograms for adults, repeated at 5 minutes if needed.
- Fluids: 500–1,000 mL in adults, 10 mL/kg in children.
- Refractory after two IM doses: adrenaline infusion and expert help.
- Observe for 2, 6 or 12 hours depending on risk. Discharge with two auto-injectors and an allergy referral.
Frequently asked questions
What is the adult dose of adrenaline in anaphylaxis?
500 micrograms intramuscularly, which is 0.5 mL of 1:1,000 adrenaline, into the anterolateral thigh. Repeat after 5 minutes if there is no improvement.
What is refractory anaphylaxis?
Anaphylaxis with persisting respiratory or cardiovascular symptoms despite two appropriate doses of IM adrenaline. It is treated with a low-dose IV adrenaline infusion and critical care support.
Are antihistamines and steroids still given in anaphylaxis?
Not as part of the initial emergency treatment. Antihistamines can be used later for skin symptoms, and steroids are reserved for selected cases after initial resuscitation.
When should mast cell tryptase be measured?
As soon as possible after treatment has started, and again ideally within 1–2 hours and no later than 4 hours from the onset of symptoms.
Next steps
Try our 20 free MRCEM SBA sample questions to test yourself. For more, explore our Resuscitation and Allergy questions and the Allergy course, or see the full question bank and mock tests in our membership plans.
This article is for exam preparation. In clinical practice, always follow the current Resuscitation Council UK anaphylaxis guidance and your local protocol.