Why Anaphylaxis Is a Core OSCE Station
Anaphylaxis is a time-critical emergency examined in acute management stations, simulation, and prescribing scenarios. Examiners want to see: correct recognition (including atypical presentations), immediate intramuscular adrenaline without delay, systematic ABCDE management, and post-crisis care including allergen identification and follow-up planning.
⚠️ Red Flag
The single most important action in anaphylaxis is early IM adrenaline. Every other intervention — antihistamines, corticosteroids, fluid — is secondary. Delayed adrenaline is the leading cause of preventable anaphylaxis deaths in the UK.
Recognition — Diagnostic Criteria
Anaphylaxis is likely when any one of the following three criteria is met:
| Criterion | Features |
|---|---|
| 1 — Sudden onset with skin/mucosal involvement PLUS respiratory or cardiovascular compromise | Urticaria + wheeze OR urticaria + hypotension |
| 2 — Two or more systems affected after allergen exposure | Any two of: skin, respiratory, cardiovascular, gastrointestinal |
| 3 — Hypotension alone after known allergen exposure | Systolic below 90 mmHg (or more than 30% drop from baseline) |
Classic features:
- Skin: urticaria, erythema, angioedema, pruritus
- Airway: throat tightening, stridor, hoarse voice
- Breathing: wheeze, dyspnoea, accessory muscle use
- Circulation: tachycardia, hypotension, syncope
- GI: nausea, vomiting, abdominal cramps
🧠 Mnemonic
SAFE — recognising anaphylaxis:
- S kin changes (urticaria, flushing, angioedema)
- A irway compromise (stridor, throat tightening)
- F alling blood pressure (tachycardia, hypotension, syncope)
- E xposure history (insect sting, food, medication, contrast)
Not all features need to be present — act early if you suspect anaphylaxis.
Immediate Management — Resuscitation Council UK Algorithm
Step 1: Call for help
"I would immediately call the resuscitation team / 2222 and state suspected anaphylaxis."
Step 2: Position
- Airway compromise dominant: upright position
- Cardiovascular compromise dominant: supine with legs elevated (increases venous return)
- Pregnant patient: left lateral tilt (30 degrees) to relieve aortocaval compression
Step 3: Adrenaline (MOST CRITICAL STEP)
| Patient | Dose | Route | Site |
|---|---|---|---|
| Adult (over 12 years) | 500 micrograms (0.5 mL of 1:1000) | Intramuscular | Anterolateral thigh |
| Child 6-12 years | 300 micrograms (0.3 mL) | IM | Anterolateral thigh |
| Child under 6 years | 150 micrograms (0.15 mL) | IM | Anterolateral thigh |
⚠️ Red Flag
Critical dose detail: Adrenaline 1:1000 (1 mg/mL) for IM use. NOT 1:10,000 (which is for IV use in cardiac arrest only). State the concentration and route explicitly to the examiner. IV adrenaline outside a monitored resuscitation setting is dangerous and has caused deaths.
Repeat adrenaline after 5 minutes if no improvement. There is no maximum number of doses — repeat every 5 minutes as needed until response.
Step 4: Oxygen and IV access
- High-flow oxygen 15 L/min via non-rebreathe mask
- Establish two large-bore IV cannulae
Step 5: IV fluid challenge
- 500 mL sodium chloride 0.9% stat (if cardiovascular compromise)
- Repeat if needed — some patients require litres
Step 6: Antihistamine (secondary — do NOT delay adrenaline for this)
- Chlorphenamine 10 mg IV/IM (adult)
- Reduces but does not treat histamine-mediated symptoms
Step 7: Corticosteroid (secondary — prevents biphasic reaction)
- Hydrocortisone 200 mg IV/IM (adult)
- Works over hours — not for acute reversal
Biphasic Anaphylaxis
A second anaphylactic reaction occurs in approximately 5-20% of cases, typically 4-12 hours after the initial event.
💎 Clinical Pearl
All patients with anaphylaxis must be observed for a minimum of 6 hours after resolution of symptoms (12 hours if severe, required resuscitation, or asthmatic). This is a mandatory OSCE safety-netting point — state it explicitly.
Allergen Identification and Post-Crisis Care
Investigations:
- Mast cell tryptase: take three samples — at the time of reaction, 1-2 hours after, and 24 hours after (baseline). A rise confirms anaphylaxis; levels peak at 1-2 hours.
- Document suspected trigger, time of exposure, and timeline of symptoms
Before discharge, always:
- 1Prescribe two adrenaline auto-injectors (EpiPen or Jext)
- 2Teach injection technique (anterolateral thigh, through clothing if needed)
- 3Counsel on allergen avoidance
- 4Provide written anaphylaxis action plan
- 5Refer to allergy clinic for formal testing
- 6Advise to wear medical alert jewellery
- 7Safety-net: return immediately if symptoms recur
Auto-Injector Counselling (Common OSCE Task)
🧠 Mnemonic
THREE steps for EpiPen use — "Blue to the sky, orange to the thigh":
- 1Pull off blue safety cap
- 2Press orange tip firmly into anterolateral thigh (can be through clothing)
- 3Hold for 10 seconds — remove and rub site
- 4Call 999 immediately after use — adrenaline wears off in 10-20 minutes
- 5Use second auto-injector if no improvement after 5 minutes
Frequently Asked Questions
"What is the correct site for IM adrenaline and why?"
The anterolateral thigh (vastus lateralis). It has better blood flow and faster absorption than the deltoid, particularly when there is vasodilation and shock. The thigh is also accessible in a collapsed patient. Do not inject into the buttock — absorption is slow and unreliable.
"What is the difference between anaphylaxis and a severe allergic reaction?"
Anaphylaxis specifically involves multi-system involvement and/or cardiovascular or airway compromise. A severe allergic reaction confined to the skin (urticaria, angioedema without airway or cardiovascular involvement) is not anaphylaxis by definition. However, skin reactions can progress — monitor closely and have a low threshold for treating as anaphylaxis.
"Can antihistamines treat anaphylaxis?"
No. Antihistamines are adjuncts — they reduce pruritus and urticaria but have no effect on the cardiovascular collapse or bronchospasm that cause mortality. Giving antihistamines instead of adrenaline is a common and dangerous error. Adrenaline must always come first.
"Why are two auto-injectors prescribed?"
Because the effect of one dose may be insufficient in a severe reaction, the second dose may be needed if symptoms persist or recur before emergency services arrive. NICE and BSACI recommend prescribing two devices as standard. Patients should carry both at all times.
"When should patients be referred to an allergy clinic?"
All patients who have experienced confirmed or suspected anaphylaxis should be referred for specialist allergy assessment. This allows formal allergen testing (skin prick testing, specific IgE), confirmation of diagnosis, further education, and consideration of allergen immunotherapy where appropriate (e.g., venom anaphylaxis).
Related Posts
- Basic Life Support OSCE — managing the unresponsive collapsed patient
- Acute Asthma Management OSCE — bronchospasm in the context of acute allergic response
- Drug History OSCE — identifying drug allergies and adverse reactions in history taking