Allergic reaction / anaphylaxis
A streamlined, differential-led presentation for rapid clinical review.
Twenty-second review
- Allergic reactions range from localized mediator release to anaphylaxis with airway edema, bronchospasm, capillary leak, and distributive shock.
- Voice change, tongue swelling, stridor, wheeze, shock, collapse, or multi-system involvement after exposure requires immediate anaphylaxis care.
- Keep asthma or vasovagal event in the differential; overlapping or concurrent illness remains possible.
- For allergic reaction / anaphylaxis, trends and combinations of findings are more informative than one isolated vital sign.
Compare the clinical patterns
The repeated material is grouped inside each possibility. Open only the condition you need.
AnaphylaxisTime-sensitive high-risk possibility
HPI
Clarify trigger and exposure time; skin findings; facial/tongue swelling.
Signs & symptoms
Airway, breathing, or circulatory involvement, sometimes without skin findings.
Focused exam
Inspect the airway, skin, face, and exposed areas; auscultate lungs and assess perfusion; Reassess frequently because swelling and shock may progress.
Brief pathophysiology
Anaphylaxis may cause allergic reaction / anaphylaxis through a time-sensitive process capable of rapid physiological deterioration.
What the trend may show
Anaphylaxis may involve airway, breathing, or circulation alone or in combination. Do not wait for hypotension, hypoxemia, or skin findings before recognizing a severe reaction.
Blood pressure (mmHg)
May fallVasodilation and capillary leak can cause rapid hypotension.
Heart rate (bpm)
Often risesTachycardia may accompany shock, distress, or epinephrine treatment.
Respiratory rate (breaths/min)
Often risesTachypnea may reflect bronchospasm, airway edema, or shock.
Oxygen saturation (SpO₂)
May fallDesaturation suggests significant respiratory involvement but may be late.
Front-load in the triage report
- Anaphylaxis: Airway, breathing, or circulatory involvement, sometimes without skin findings.
- Trigger and exposure time.
- Skin findings.
Localized allergic reactionCommon presentation pattern
HPI
Clarify trigger and exposure time; skin findings; facial/tongue swelling.
Signs & symptoms
Urticaria or itching without respiratory or circulatory involvement may remain limited.
Focused exam
Inspect the airway, skin, face, and exposed areas; auscultate lungs and assess perfusion; Reassess frequently because swelling and shock may progress.
Brief pathophysiology
Localized allergic reaction may explain the presenting symptoms through a common clinical mechanism, while serial assessment checks for complications or an alternate cause.
What the trend may show
A localized reaction generally preserves breathing and circulation. Any airway symptom, respiratory compromise, or hypotension should trigger reassessment for anaphylaxis.
Blood pressure (mmHg)
Usually preservedFalling pressure is not expected in a localized reaction.
Respiratory rate (breaths/min)
Usually normalNew respiratory distress suggests systemic involvement.
Oxygen saturation (SpO₂)
Usually normalDesaturation suggests respiratory involvement or another condition.
Heart rate (bpm)
Usually near baselinePain or anxiety may increase rate; persistent tachycardia warrants reassessment.
Front-load in the triage report
- Urticaria or itching without respiratory or circulatory involvement may remain limited.
- Trigger and exposure time.
- Skin findings.
AsthmaImportant alternative presentation
HPI
Clarify trigger and exposure time; skin findings; facial/tongue swelling.
Signs & symptoms
Respiratory or collapse symptoms may overlap but exposure and multisystem signs guide concern.
Focused exam
Inspect the airway, skin, face, and exposed areas; auscultate lungs and assess perfusion; Reassess frequently because swelling and shock may progress.
Brief pathophysiology
Asthma can resemble allergic reaction / anaphylaxis through overlapping symptoms or referred physiology despite a different primary mechanism.
What the trend may show
Respiratory rate and effort are primary. A falling rate with fatigue, reduced air entry, or altered consciousness can signal impending ventilatory failure rather than improvement.
Respiratory rate (breaths/min)
Often risesTachypnea is common; a late fall with exhaustion is ominous.
Oxygen saturation (SpO₂)
May fallHypoxemia can occur in severe bronchospasm, but ventilation and fatigue also require assessment.
Heart rate (bpm)
Often risesDistress and beta-agonists may increase heart rate.
Blood pressure (mmHg)
Usually preservedHypotension is a late or peri-arrest feature and warrants urgent reassessment.
Front-load in the triage report
- Respiratory or collapse symptoms may overlap but exposure and multisystem signs guide concern.
- Trigger and exposure time.
- Skin findings.
Vasovagal eventImportant alternative presentation
HPI
Clarify skin findings; facial/tongue swelling; voice change/stridor.
Signs & symptoms
Respiratory or collapse symptoms may overlap but exposure and multisystem signs guide concern.
Focused exam
Reassess frequently because swelling and shock may progress; Inspect face, tongue, mucosa, skin, and exposure site; assess air entry, wheeze, stridor, GI symptoms, and perfusion.
Brief pathophysiology
Vasovagal event can resemble allergic reaction / anaphylaxis through overlapping symptoms or referred physiology despite a different primary mechanism.
What the trend may show
Anaphylaxis may involve airway, breathing, or circulation alone or in combination. Do not wait for hypotension, hypoxemia, or skin findings before recognizing a severe reaction.
Blood pressure (mmHg)
May fallVasodilation and capillary leak can cause rapid hypotension.
Heart rate (bpm)
Often risesTachycardia may accompany shock, distress, or epinephrine treatment.
Respiratory rate (breaths/min)
Often risesTachypnea may reflect bronchospasm, airway edema, or shock.
Oxygen saturation (SpO₂)
May fallDesaturation suggests significant respiratory involvement but may be late.
Front-load in the triage report
- Respiratory or collapse symptoms may overlap but exposure and multisystem signs guide concern.
- Skin findings.
- Facial/tongue swelling.
Findings that should escalate concern
- Voice change, tongue swelling, stridor, wheeze, shock, collapse, or multi-system involvement after exposure requires immediate anaphylaxis care.
- Anaphylaxis: Airway, breathing, or circulatory involvement, sometimes without skin findings.
Medication recognition
Continue the clinical pathway
Clinical and academic sources
Curated clinical guidance and peer-reviewed evidence supporting this presentation. Current BCEHS guidance, medical direction, and local policy remain authoritative for practice.
BCEHS Handbook — specific CPGs
- E09: AnaphylaxisBC Emergency Health Services Handbook
Official BCEHS clinical practice guidance for anaphylaxis.
- E10: Minor AllergyBC Emergency Health Services Handbook
Official BCEHS clinical practice guidance for minor allergy.
Supporting clinical and academic evidence
- CEDIS Presenting Complaint List updateCanadian Association of Emergency Physicians
Presenting-complaint terminology and categorization context. It does not establish a prehospital “top 100” ranking.
- World Allergy Organization anaphylaxis guidance 2020World Allergy Organization
Peer-reviewed international guidance on recognizing and managing anaphylaxis.
- EAACI guidelines: Anaphylaxis (2021 update)Allergy · 2022
Anaphylaxis recognition, first-line epinephrine, observation, and distinction from less severe allergic reactions.