Allergic Reaction
ABCs & initial assessment
- airway: stridor, voice change, tongue / lip swelling → basic maneuvers, BVM if needed, call ICU / ENT early, prepare for intubation
- breathing: wheeze, dyspnea, hypoxia → high-flow O₂, nebulized salbutamol, consider epinephrine
- circulation: hypotension, tachycardia → IV access, 500–1,000 mL crystalloid, continuous monitoring
- disability: confusion, dizziness → hypoperfusion / hypoxia, reassess frequently
- exposure: urticaria, angioedema → check for systemic involvement
assume anaphylaxis until proven otherwise, give epinephrine early
escalate early
- persistent hypotension despite fluids + epinephrine
- worsening airway compromise
- biphasic reaction → monitor 6–24 hr
- on discharge: adrenaline auto-injector + allergy/immunology referral
- ACEi angioedema → often refractory; airway focus + ICU early
diagnostic criteria
acute onset + any one of:
- known allergen + hypotension (SBP < 90 mmHg or ↓ ≥ 30% baseline)
- skin / mucosa + ≥ 1 system (cardio: hypotension, syncope; resp: wheeze, stridor, dyspnea)
- suspected allergen + ≥ 2 systems
- skin → urticaria, angioedema
- respiratory → wheeze, stridor
- GI → vomiting, diarrhea
- cardiovascular → hypotension, collapse
HPI & examination
- history: trigger (food, drugs, insect stings, environment); prior reactions/allergies; medications (antibiotics, NSAIDs, contrast)
- examination: skin (urticaria, angioedema); airway (hoarseness, stridor, swelling); resp (wheeze, distress); cardio (hypotension, tachycardia); neuro (dizziness, syncope)
management
anaphylaxis
- immediate treatment
- IM Epinephrine 0.5 mg (1:1000) mid-thigh; repeat q 5 minutes if airway/breathing/circulation not improving
- lay flat, legs raised; upright posture can precipitate arrest
- high-flow oxygen; IV fluids → 500–1,000 mL rapid NS bolus, reassess after each, repeat as needed; smaller boluses if fluid-overload risk
- adjuncts
- nebulized salbutamol (bronchospasm)
- antihistamines: Cetirizine 10 mg PO or Chlorpheniramine 10 mg IV
- corticosteroids: not routinely recommended for anaphylaxis (Resus Council UK 2021); if specifically indicated (refractory, concurrent asthma), hydrocortisone 200 mg IV
- adjuncts do not prevent biphasic reactions; never delay or replace epinephrine
- stop offending agent; continuous monitoring; watch for biphasic reaction
investigations (do not delay treatment)
| test | why |
|---|---|
| serum tryptase | confirms diagnosis (1–3 hr peak) |
| ABG / VBG | respiratory distress |
| ECG ± troponin | persistent hypotension |
| bloods / crossmatch | if unstable / preparing for escalation |
moderate (multi-system, not anaphylaxis)
- add steroids; nebulized bronchodilators if needed
mild (isolated symptoms)
- observe; oral antihistamines: Cetirizine 10 mg, Loratadine 10 mg
escalation
- refractory → adrenaline infusion + ICU (± vasopressors); glucagon 1–2 mg IV if on beta-blocker
- airway compromise → early intubation
- unclear diagnosis → treat as anaphylaxis
evidence
| source | type |
|---|---|
| 2021 Resuscitation Council UK anaphylaxis update | international guideline |
| 2023 AAAAI/ACAAI anaphylaxis practice parameter | international guideline |
Verify indication, dose, allergies, interactions, renal/hepatic function and local protocols before prescribing.
free the on-call checklist covers the whole shift on one printable page.
studying for the IM exam? the IM Rapid Review covers anaphylaxis in the same format. see the sample chapter.
reviewed Jun 2026updated Sep 2026file complaint/allergic-reaction