About This Page
This is a clinician-written, evidence-based guide aligned to the MCC Examination Objectives. It is structured by clinical presentation — the way the MCCQE tests and the way patients actually present. Management reflects current Canadian guidelines (CMA, CFPC, CPS). Always cross-reference with institutional protocols and clinical judgment.
The Bottom Line
- Anaphylaxis is a clinical diagnosis — do not wait for tryptase or rash if airway, breathing, circulation, or multi-system involvement is present
- IM epinephrine into the mid-anterolateral thigh is first-line and life-saving; antihistamines and steroids are adjuncts, not substitutes
- Skin findings may be absent in severe anaphylaxis; hypotension, bronchospasm, stridor, vomiting, or collapse after exposure is enough to treat
- Observe after treatment because biphasic reactions occur; duration depends on severity, response, comorbidity, and access to emergency care
- Discharge planning includes epinephrine auto-injector prescription, education, trigger avoidance, written action plan, and allergy referral when appropriate
Approach to the Presentation
The key MCCQE1 distinction is simple allergic reaction versus anaphylaxis. Localized urticaria or mild pruritus without airway, breathing, circulation, or GI involvement can be managed with antihistamines and observation. Anaphylaxis is probable when symptoms are acute and involve skin/mucosa plus respiratory compromise or hypotension; two or more systems after likely allergen exposure; or hypotension/bronchospasm/laryngeal involvement after known allergen exposure. Common triggers include foods, medications, insect stings, latex, immunotherapy, and vaccines. Assess airway early, call for help, give IM epinephrine promptly, place the patient supine with legs elevated if tolerated, and establish IV access for fluids if hypotensive.
Differential Diagnosis
| diagnosis | likelihood | key features | distinguishing test |
|---|---|---|---|
| Anaphylaxis | must-not-miss | Acute onset after exposure with urticaria/angioedema, wheeze/stridor, hypotension, syncope, vomiting/diarrhoea; skin signs may be absent | Clinical diagnosis; response to IM epinephrine supports; tryptase may support later but is not required |
| Upper airway angioedema | must-not-miss | Tongue/lip/laryngeal swelling, voice change, dysphagia, stridor; may lack urticaria or pruritus | Clinical airway assessment; C4/C1 inhibitor studies later if hereditary suspected |
| Severe asthma exacerbation | must-not-miss | Wheeze, dyspnea, chest tightness, poor air entry; may coexist with anaphylaxis | Clinical; peak flow if stable; treat anaphylaxis with epinephrine if allergen/multisystem features |
| Septic shock | must-not-miss | Fever/hypothermia, infection source, hypotension, altered mentation, no clear allergen trigger | Cultures/lactate/source evaluation; distinction may be difficult in early shock |
| Vasovagal syncope | common | Pallor, bradycardia, nausea, diaphoresis after needle/procedure, rapid recovery supine, no urticaria/wheeze | Vitals show bradycardia rather than tachycardia; no multi-system allergic features |
| Panic attack / hyperventilation | common | Dyspnea sensation, tingling, chest tightness, fear, normal oxygen saturation, no objective wheeze/hypotension | Clinical after excluding anaphylaxis/asthma/PE where relevant |
| Acute urticaria without anaphylaxis | common | Itchy wheals only, no airway/breathing/circulation/GI involvement, stable vitals | Clinical; monitor for progression |
| Food poisoning or gastroenteritis | less common | Vomiting/diarrhoea after food but no urticaria, wheeze, angioedema, hypotension; multiple affected people | Clinical epidemiology; treat as anaphylaxis if multi-system allergic features |
| Scombroid poisoning | rare | Flushing, headache, palpitations, urticaria-like symptoms after fish, peppery taste, multiple affected diners | Clinical; responds to antihistamines but can mimic allergy |
| Mast cell activation disorder | rare | Recurrent unexplained flushing, urticaria, hypotension, GI symptoms, triggers variable | Tryptase during episode and baseline; allergy/immunology referral |
Red Flags & Key History
Symptoms
Voice change, tongue swelling, throat tightness, stridor, or dysphagia
Wheeze, dyspnea, hypoxia, or persistent cough after allergen exposure
Syncope, hypotension, collapse, confusion, or severe weakness
Repeated vomiting, abdominal pain, or diarrhoea after likely allergen exposure
Previous anaphylaxis, asthma, beta-blocker use, remote location, or delayed epinephrine
Itchy localized hives without systemic symptoms
Needle-triggered pallor/bradycardia with rapid recovery supine
Signs
Hypotension, tachycardia, poor perfusion, altered mental status
Stridor, hoarseness, drooling, tongue/lip swelling
Wheeze, reduced air entry, hypoxia
Generalized urticaria, flushing, angioedema
Bradycardia and pallor without rash/wheeze
Approach to Investigation
First-line
Clinical diagnosis and serial vitalsDo not delay epinephrine for tests. Monitor BP, HR, oxygen saturation, respiratory effort, airway progression, and response to therapy
Airway assessmentVoice, stridor, tongue/laryngeal swelling, ability to handle secretions; call anesthesia/ENT early if worsening
Glucose and ECG if collapseUseful in undifferentiated collapse, older adults, or unclear diagnosis
Second-line
Serum tryptaseCan support diagnosis if drawn within a few hours and compared with baseline, but should never delay treatment
Trigger assessmentMedication, vaccine, food, sting, exercise, and cofactor history; document timing and all exposures
Specialist
Allergy/immunology referralFood, venom, drug, idiopathic, recurrent, occupational, or severe anaphylaxis; plan testing after recovery
Airway specialistProgressive tongue/laryngeal oedema, stridor, ACE inhibitor angioedema, or failed response to treatment
Management Principles
Canadian anaphylaxis emergency treatment principles1
Immediate anaphylaxis treatment
- Give IM epinephrine 0.01 mg/kg of 1 mg/mL solution to a maximum adult dose of 0.5 mg into the mid-anterolateral thigh; repeat every 5-15 minutes as needed
- Call for help/EMS, place supine with legs elevated if tolerated, and avoid sudden standing
- High-flow oxygen for respiratory symptoms or hypoxia
- Two large-bore IVs and rapid isotonic crystalloid boluses for hypotension
2
Adjuncts after epinephrine
- Inhaled salbutamol for persistent bronchospasm
- Antihistamines for urticaria/pruritus only after epinephrine; they do not treat airway oedema or shock
- Corticosteroids may be used as adjuncts, but they are delayed-onset and must not replace epinephrine
- Glucagon may be considered for refractory anaphylaxis in beta-blocked patients
3
Observation and discharge
- Observe based on severity, repeated epinephrine requirement, asthma, hypotension, distance from care, and comorbidity
- Prescribe epinephrine auto-injectors and teach demonstration technique
- Provide written action plan, trigger avoidance advice, MedicAlert consideration, and allergy referral
4
Non-anaphylactic allergic reaction
- For isolated urticaria/pruritus with stable vitals and no systemic features: oral non-sedating antihistamine and observation/safety-netting
- Escalate immediately if respiratory, cardiovascular, or GI features develop
Complications & Pitfalls
- Delayed epinephrine: The commonest dangerous error is trying antihistamines first in true anaphylaxis.
- No rash trap: Severe anaphylaxis can occur without skin signs.
- Upright collapse: Keep hypotensive patients supine if tolerated.
- Incomplete discharge planning: Treatment is incomplete without auto-injector training, trigger documentation, and follow-up.
- ACE inhibitor angioedema: May not respond to epinephrine/antihistamines; airway management is the priority.
MCCQE1 Exam Tips
- 1The next best step in anaphylaxis is IM epinephrine — not antihistamines, steroids, tryptase, or observation
- 2Use the mid-anterolateral thigh route; subcutaneous epinephrine is not preferred emergency treatment
- 3Anaphylaxis can present as hypotension and bronchospasm without rash
- 4Vasovagal after vaccine/needle: bradycardia and pallor; anaphylaxis: tachycardia, urticaria/wheeze/angioedema/hypotension
- 5Discharge after anaphylaxis requires epinephrine auto-injector, teaching, action plan, and allergy follow-up
- 6CanMEDS communicator role: document the trigger clearly and teach patient/family when and how to use epinephrine
- 7Beta-blocked patient with refractory anaphylaxis: consider glucagon as an adjunct after epinephrine and fluids
practicetest your knowledge on allergic reaction & anaphylaxisApply what you've learnt with MCCQE1-style questions from the iatroX Q-Bank — general & constitutional and beyond.
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