Anaphylaxis Algorithm · Reasoning GP

Anaphylaxis = acute onset + life-threatening Airway/Breathing/Circulation problem. IM adrenaline is the only first-line treatment — give it early, repeat at 5 min. StartDecisionInvestigateActionReferStop / Admit

Start · Recognise (Resus Council UK 2021)Acute onset + life-threatening A / B / C problem

Sudden onset and rapid progression of symptoms plus a life-threatening Airway (swelling, hoarseness, stridor), Breathing (wheeze, ↑RR, cyanosis, SpO₂ <94%, fatigue) or Circulation (pale/clammy, tachycardia, low BP, faint, collapse) problem. Skin/mucosal changes (flushing, urticaria, angioedema) usually present but subtle or absent in 20% — and skin changes alone are not anaphylaxis.

Decision · Safety firstIs this anaphylaxis?

A–B–C–D–E assessment. GI symptoms (vomiting, abdominal pain, incontinence) may occur. Asthma and anaphylaxis overlap — treat for anaphylaxis if any life-threatening feature.

YES — anaphylaxis

Stop · Treat nowIM adrenaline + 999

Give IM adrenaline (1:1000) into the anterolateral thigh immediately, call 999 and summon help. Position the patient; remove the trigger (e.g. sting). Repeat adrenaline after 5 min if no improvement.

NO life-threatening feature

ReassessSkin-only / allergic reaction

Skin changes with no airway/breathing/circulation compromise are not anaphylaxis — observe, treat symptomatically (non-sedating oral antihistamine for skin), and safety-net.

IM adrenaline 1:1000 — dose by age

Adult & child >12y

500 µg (0.5 mL)

IM anterolateral thigh.

Child 6–12y

300 µg (0.3 mL)

IM anterolateral thigh.

Child 6 months–6y

150 µg (0.15 mL)

(<6 months: 100–150 µg / 0.1–0.15 mL.)

supportive measures

Action · Position & supportPosition, oxygen, fluids

Lie flat with legs raised if hypotensive (semi-recumbent if airway/breathing difficulty). Do not sit or stand the patient up — sudden standing can be fatal.
Establish airway; high-flow oxygen; monitor SpO₂, BP, ECG (don't delay treatment to attach monitoring).
If no response: IV fluid challenge (crystalloid — adult 0.5–1 L; child 10 mL/kg) when skills/equipment allow.
Do NOT give antihistamines or steroids as part of acute resuscitation — ineffective; hydrocortisone is no longer recommended. Antihistamines only later, for residual skin symptoms.

no response after 2 IM doses

Step 4 · Refractory anaphylaxisStill no improvement after ≥2 IM doses

Refractory anaphylaxis = ABC features persisting despite 2 appropriate IM adrenaline doses. Give a rapid IV fluid bolus and start a low-dose IV adrenaline infusion — by those trained/equipped only, with continuous monitoring (ALS / critical-care / 999 paramedic support). Confirm the diagnosis and basics first: high-flow O₂, fluids, trigger removed, correct IM dose/site.

confirm & observe

Step 5 · Confirm — mast cell tryptaseTimed serum tryptase

Take timed samples — ideally one ASAP after resuscitation starts, a second 1–2 h after symptom onset, and a baseline ≥24 h later / at follow-up. A transient rise supports mast-cell degranulation and confirms the event for the later allergy work-up.

Step 6 · Refer — admit & observeBiphasic-reaction risk

Admit all for observation — biphasic reactions can recur hours later without re-exposure. Observation length depends on severity, response and reaction pattern (Resus Council UK).

before discharge

Step 7–8 · Auto-injectors & allergy referralTwo AAIs + specialist

Prescribe two adrenaline auto-injectors, demonstrate the exact device, give a written action plan, and refer to allergy/immunology to identify the trigger and plan long-term care.

long-term

Step 9 · Avoidance & safety-netCarry two, always

Carry both AAIs at all times, avoid the known trigger, consider a medical-alert identifier, and always call 999 + attend hospital after any AAI use even if symptoms settle.

⚠️ IM adrenaline is the only first-line drug — give it early into the anterolateral thigh and repeat after 5 minutes if needed. Never delay it for antihistamines or steroids (no role in resuscitation). Keep the patient lying down; sitting or standing up suddenly can cause fatal cardiovascular collapse.

Safety

Recognising Anaphylaxis

Done

Anaphylaxis is likely when all three are present: acute onset, rapid progression, and a life-threatening Airway, Breathing or Circulation problem. Skin changes are usually but not always present.

Airway Swelling, hoarseness, stridor, sensation the throat is closing, difficulty breathing/swallowing.

Breathing Increased respiratory rate, wheeze, fatigue, cyanosis, SpO₂ <94%.

Circulation Pale, clammy, tachycardia, low BP, feeling faint, confusion, collapse.

Skin / mucosa (supportive) Flushing, urticaria, angioedema — subtle or absent in ~20%. Skin changes alone are NOT enough to diagnose anaphylaxis. GI symptoms (vomiting, pain, incontinence) may occur.

The Resuscitation Council UK definition deliberately anchors the diagnosis on a life-threatening A, B or C problem rather than on the rash, because up to one in five anaphylactic reactions have minimal or no skin involvement — waiting for urticaria before treating risks fatal delay. Asthma and anaphylaxis overlap and can be hard to separate in the moment; if there is any life-threatening feature with a plausible trigger, treat for anaphylaxis.

Treat

IM Adrenaline — First-Line, Immediately

Done

Give IM adrenaline (1:1000) into the anterolateral thigh as soon as anaphylaxis is recognised. Call 999 and summon help. Repeat after 5 minutes if there is no improvement.

Adult & child >12y

500 µg = 0.5 mL of 1:1000 (1 mg/mL) IM.

Child 6–12y

300 µg = 0.3 mL IM.

Child 6 months–6y

150 µg = 0.15 mL IM.

Child <6 months

100–150 µg = 0.1–0.15 mL IM.

Route & repeat

Anterolateral thigh; repeat every 5 minutes as needed pending response. IV adrenaline is for specialists only (titrated, monitored).

Adrenaline reverses the pathophysiology of anaphylaxis — it is a vasoconstrictor (restoring blood pressure), a bronchodilator, and it stabilises mast cells. The intramuscular anterolateral thigh route gives rapid, reliable absorption with a wide safety margin, whereas IV adrenaline in untrained hands causes arrhythmia and is reserved for specialists. The commonest cause of a fatal outcome is delayed or omitted adrenaline.

Treat

Position & Supportive Care

Done

Position

Lie flat with legs raised if hypotensive; semi-recumbent if airway/breathing difficulty makes that more comfortable. Do not let the patient stand or sit up suddenly — fatal collapse can occur within minutes. Remove the trigger (e.g. stop an infusion, remove a sting).

Airway & oxygen

Establish/maintain the airway; give high-flow oxygen. Monitor SpO₂, BP and ECG — but do not delay adrenaline to set up monitoring.

Fluids

If no response to adrenaline: IV crystalloid fluid challenge — adult 0.5–1 L, child 10 mL/kg — repeated as needed.

What NOT to give

Antihistamines and corticosteroids have no role in acute resuscitation (ineffective; hydrocortisone no longer recommended routinely). A non-sedating oral antihistamine may be used after stabilisation for residual skin symptoms only.

Posture is a genuinely life-saving and under-appreciated intervention: in the vasodilated, fluid-depleted circulation of anaphylaxis, suddenly sitting or standing can cause an "empty ventricle" and cardiac arrest, so patients are kept supine with legs raised. The 2021 guidance removed routine antihistamines and steroids from the acute algorithm because they neither treat the airway/circulatory collapse nor reliably prevent biphasic reactions, and giving them can distract from, or delay, repeat adrenaline.

Treat

Refractory Anaphylaxis

Done

Refractory anaphylaxis = persisting respiratory or cardiovascular compromise despite two appropriate doses of IM adrenaline. This is a peri-arrest emergency — escalate to the highest level of help available (999 / critical care / ALS team).

Recheck the basics

Confirm the diagnosis, that the IM dose, site and route were correct, the trigger is removed, and high-flow oxygen + an IV fluid bolus have been given.

IV adrenaline infusion

Start a low-dose IV adrenaline infusion — by clinicians trained and equipped to use it, with continuous ECG, SpO₂ and BP monitoring. Bolus IV adrenaline is for specialists/cardiac arrest only.

Second-line agents

Consider per specialist advice (e.g. glucagon if on a beta-blocker and not responding to adrenaline). Persisting bronchospasm → treat as severe asthma.

If cardiac arrest

Start ALS immediately; prolonged resuscitation may be appropriate.

A minority of reactions do not respond to repeated IM adrenaline; the 2021 guidance formalised "refractory anaphylaxis" precisely so that clinicians escalate to a controlled IV adrenaline infusion and aggressive fluid resuscitation rather than simply repeating IM doses indefinitely. Patients on beta-blockers may be resistant to adrenaline, which is why glucagon is held in reserve. The infusion route demands monitoring and trained hands because uncontrolled IV adrenaline causes arrhythmia and myocardial ischaemia.

Diagnose

Confirm the Diagnosis — Mast Cell Tryptase

Done

Anaphylaxis is a clinical diagnosis — never delay adrenaline to take bloods — but timed tryptase samples confirm it afterwards and underpin the allergy work-up.

Sample 1

As soon as feasible after resuscitation has started (do not interrupt treatment).

Sample 2

1–2 hours after symptom onset — when tryptase peaks.

Baseline

A later sample ≥24 h after full recovery or at follow-up, to establish the patient's baseline (a persistently raised baseline suggests mastocytosis).

Interpretation

A transient rise then fall supports mast-cell activation. A normal tryptase does not exclude anaphylaxis (especially food-triggered/normotensive reactions).

Tryptase is released when mast cells degranulate and peaks 1–2 hours after the reaction, so a single level is hard to interpret — it is the dynamic change between an acute sample and a recovered baseline that confirms anaphylaxis. The baseline also screens for mastocytosis, an important underlying cause of recurrent or severe reactions. Crucially, a normal tryptase never overrides a convincing clinical picture, particularly in food-triggered reactions where levels often stay normal.

Refer

Admission & Biphasic Observation

Done

Admit everyone

All patients treated for anaphylaxis need a period of observation — a biphasic reaction can recur hours after apparent recovery, without re-exposure to the trigger.

Observation period

Length is risk-stratified by severity, speed of response and reaction pattern (per Resus Council UK) — longer if severe, slow to respond, prior biphasic reaction, ongoing asthma, or unable to access emergency care quickly.

Lower threshold to keep in

Reactions needing >1 adrenaline dose, severe asthma component, possible ongoing absorption (e.g. ingested allergen), evening/night presentation, or living alone / remote from help.

Children

Children who have had anaphylaxis should be admitted under paediatric care for observation.

A biphasic reaction is a second wave of anaphylaxis occurring after complete resolution of the first, typically within hours and without further allergen exposure; because it can be as severe as the index event, observation is mandatory even when the patient looks fully recovered. The observation duration is deliberately risk-based rather than a fixed number, concentrating monitoring on those most likely to deteriorate again.

Treat

Discharge — Two Adrenaline Auto-Injectors

Done

Prescribe two

Everyone who has had anaphylaxis is discharged with two adrenaline auto-injectors (AAIs) and told to carry both, on their person, at all times.

Demonstrate the device

Show how to use their specific brand (designs differ); have the patient/carer demonstrate back. Avoid switching brands without re-training.

Dose by device

Adult / child ≥12y (and many 6–12y) → 300 µg AAI; smaller children → 150 µg AAI per weight. (300 µg AAI delivers less than the 500 µg IM resuscitation dose — hence call 999 too.)

Replace & check

Note expiry dates; replace used or expired devices promptly. Register for expiry-alert services where available.

Two injectors are prescribed because a single device may misfire, be used incorrectly, or be insufficient if symptoms recur before the ambulance arrives — and the fixed AAI dose (usually 300 µg) is lower than the 500 µg given under medical supervision, so a second dose is frequently needed. Hands-on, brand-specific training matters because the commonest reason an injector fails to help in a real event is incorrect technique under stress.

Refer

Specialist Allergy / Immunology Referral

Done

Refer everyone

NICE CG134: refer all patients after a suspected anaphylactic reaction to a specialist allergy service to identify the trigger and reduce the risk of further reactions.

What the clinic adds

Allergen testing (skin-prick / specific IgE), confirmation of the trigger, advice on cross-reactivity, and where appropriate immunotherapy (e.g. venom, some foods).

Document the suspected trigger

Record the food/drug/insect, timing and co-factors (exercise, alcohol, NSAIDs) on the referral; flag drug culprits as an allergy in the record to prevent re-exposure.

Interim advice

Strict avoidance of the suspected trigger until specialist review; carry both AAIs meanwhile.

Identifying the precise trigger is the single most effective long-term intervention, because reliable avoidance prevents recurrence — and specialist testing also uncovers cross-reacting allergens and co-factors (exercise, NSAIDs, alcohol) that the patient could not deduce alone. For venom and selected food allergies, immunotherapy offered through these services can be disease-modifying, which is why NICE mandates referral of everyone, not just severe cases.

Follow-up

Avoidance, Action Plan & Safety-net

Done

Avoid the trigger Educate on reading food labels, declaring allergies when eating out, and avoiding the culprit drug/insect; address co-factors (exercise, NSAIDs, alcohol).

Written action plan Personalised plan: recognise symptoms, give adrenaline early, call 999, lie flat with legs raised. Share with school/workplace/carers.

Medical-alert ID Consider a bracelet/identifier naming the allergy, so responders act fast if the patient can't speak.

After any AAI use Always call 999 and attend hospital even if symptoms improve — a second device may be needed and biphasic reactions occur.

GP follow-up

Confirm the allergy is coded, AAIs are in date and the patient/carers remain confident with technique; re-train at reviews. Ensure the specialist referral has been actioned.

Return / 999

Any recurrence of airway, breathing or circulation symptoms — give adrenaline and call 999.

An auto-injector is only useful if the patient and those around them recognise the reaction, reach the device, and use it correctly and early — so a written action plan, shared with school or workplace, and regular technique re-checks are as important as the prescription itself. Coding a drug culprit as a formal allergy in the record is a specific safety step that prevents fatal re-prescription.