01OverviewDefinition, clinical context and the essential points that orientate the chapter.
Anaphylaxis is a rapidly evolving systemic hypersensitivity reaction that threatens the airway, breathing or circulation. Urticaria and mucosal swelling can support recognition but their absence does not make severe circulatory or respiratory compromise safe. Conversely, isolated itchy skin without ABC involvement does not automatically represent anaphylaxis. Treatment follows the physiological threat, not the amount of visible rash.
Adrenaline addresses several components at once: vascular tone, capillary leak, bronchospasm and cardiac support. Delaying it while giving an antihistamine leaves the principal threats untreated. Emergency treatment and later diagnostic investigation are separate tasks; the history and timed tryptase samples help future evaluation but must not defer resuscitation.
Key points
- Anaphylaxis is a clinical diagnosis; skin changes may be absent in severe reactions.
- Give adult adrenaline 500 micrograms IM using 0.5 mL of 1 mg/mL solution.
- Repeat IM adrenaline after five minutes when airway, breathing or circulation compromise persists.
- Keep the patient lying down where possible; allow supported sitting for difficult breathing and avoid sudden standing.
- Persistent respiratory or circulatory problems despite two appropriate IM doses require the refractory-anaphylaxis pathway and expert support.
- Use NICE NG258 risk-based observation from symptom resolution, then ensure allergy referral and an effective discharge safety plan.
02AetiologyUnderlying causes, associations and risk factors, with why each one matters.
Allergen exposure
Foods, insect venom and medicines can provoke anaphylaxis; the relevant exposure and timing differ by age, setting and previous sensitisation history.
Other activation pathways
Not every reaction is driven by allergen-specific IgE, so a previous uneventful exposure or lack of a known allergy does not exclude anaphylaxis.
03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.
- 1Mediator release
Mast-cell and related mediator release alters vascular permeability, smooth-muscle tone and mucosal tissues, producing rapidly changing effects across several organ systems.
- 2Intravascular depletion
Vasodilatation and leakage of fluid from the circulation reduce venous return and cardiac filling, helping explain collapse and the danger of sudden upright positioning.
- 3Airway and bronchial effects
Oedema can narrow the upper airway while bronchial smooth-muscle contraction reduces lower-airway flow, creating different problems that may occur simultaneously.
04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.
Tongue or throat swelling, altered voice and stridor suggest upper-airway oedema. Obtain skilled airway help early because progressive swelling can make later airway intervention substantially harder.
Wheeze, respiratory distress, cyanosis or exhaustion can develop rapidly. An apparently quiet chest with poor air movement is concerning, particularly with a falling conscious level.
Hypotension, collapse and confusion can occur with little visible skin change. Reduced venous return makes sudden sitting or standing dangerous even after subjective improvement.
Ask about foods, stings, injected medicines, contrast, latex and perioperative exposures. Several agents may have been given close together, so document timing without assigning a definitive culprit prematurely.
05InvestigationsWhat to request, why it matters and how to interpret it.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Clinical assessment and continuous observationsFirst step - Why
- Determine severity and judge the immediate treatment response.
- Interpretation and limitations
- Measure pressure, pulse, oxygen saturation and respiratory effort repeatedly. There is no bedside test that must become positive before adrenaline is justified.
- 02
Timed mast-cell tryptase in adults - Why
- Support later confirmation of mast-cell activation after treatment begins.
- Interpretation and limitations
- Take a sample as soon as possible after emergency treatment has started and another ideally one to two hours, no later than four hours, after symptom onset.
- 03
Blood gas and routine blood sampling - Why
- Assess severe shock or respiratory compromise when clinically indicated.
- Interpretation and limitations
- A gas can reveal acidaemia and impaired ventilation. These tests quantify consequences but a normal result does not exclude a treated or predominantly airway reaction.
- 04
Specialist allergy assessment and baseline tryptase - Why
- Clarify the trigger and plan future avoidance and protection.
- Interpretation and limitations
- Arrange specialist referral after suspected anaphylaxis; a later baseline tryptase helps interpret acute results. Testing must be matched to the exposure history, not an indiscriminate allergy panel.
06Differential diagnosisRealistic alternatives and the features that help distinguish them.
Vasovagal collapse
A trigger such as a needle can cause transient pallor, bradycardia and hypotension; ongoing respiratory compromise or progressive swelling argues against a simple faint.
Acute asthma
Asthma can cause severe wheeze and hypoxaemia, but new allergen-associated hypotension or upper-airway swelling makes anaphylaxis a more comprehensive explanation.
Bradykinin-mediated angioedema
ACE-inhibitor or hereditary angioedema can threaten the airway without urticaria; urgent airway assessment remains necessary while specialist treatment addresses the different mechanism.
07ManagementImmediate care, first-line treatment, alternatives and escalation.
01Immediate treatmentAdrenaline and physiological supportFirst stepRapidly developing airway, breathing or circulation problems make anaphylaxis the working diagnosis.+
- 1Stop the suspected trigger when feasible, call help and give 500 micrograms IM adrenaline into the anterolateral thigh without waiting for venous access.
- 2Position safely, provide oxygen and airway support, attach monitoring and obtain IV access; give rapid crystalloid for hypotension, shock or poor initial response.
- 3Reassess at five minutes and repeat IM adrenaline if ABC problems persist. Antihistamines do not treat these threats and corticosteroids are not routine emergency treatment.
02Refractory reactionPersistent compromise after two IM dosesRespiratory or circulatory compromise continues despite two correctly delivered intramuscular adrenaline doses.+
- 1Call the critical-care or anaesthetic team and confirm dose, concentration, injection site and timing while continuing airway and circulatory support.
- 2Prepare a titrated IV adrenaline infusion for an experienced specialist using controlled delivery and continuous monitoring; cardiac-arrest experience alone is insufficient qualification for IV anaphylaxis dosing.
- 3Continue IM treatment as needed while specialist infusion support is being established, reassess fluid requirements and seek other causes of persistent instability.
03Observation and dischargeApplying the 2026 risk-based observation periodsAirway swelling has resolved, breathing has normalised and blood pressure and heart rate are stable.+
- 1A suitably qualified, experienced clinician may consider discharge after two hours from resolution only with good response within 5–10 minutes to one IM adrenaline dose given within 30 minutes of symptom onset, complete recovery, two already available in-date injectors with demonstrated knowledge, and adequate supervision when needed.
- 2Observe at least six hours after resolution when two IM doses were required or there is previous biphasic anaphylaxis; use at least twelve hours for severe reactions or specified higher-risk circumstances.
- 3At least twelve hours applies to more than two adrenaline doses, severe asthma or respiratory compromise, continuing allergen absorption, out-of-hours presentation, impaired ability to respond or difficult emergency access.
- 4Before discharge, provide a written action plan, brand-specific injector demonstration, avoidance advice and allergy referral; ensure two in-date injectors unless the cause is an easily avoided drug allergy.
Key medicines and prescribing safety2 treatments · regimens, roles and cautions+
Adrenaline for adult anaphylaxis
Give 500 micrograms IM, equal to 0.5 mL of 1 mg/mL adrenaline, into the anterolateral thigh; repeat after 5 minutes if airway, breathing or circulation problems persist.Check concentration and route explicitly. Avoid routine IV bolus adrenaline in a patient with a circulation; use specialist titrated IV treatment for refractory reactions.
Non-glucose isotonic crystalloid
For adult hypotension, shock or poor response to adrenaline, give a rapid 500–1,000 mL IV bolus and reassess; further fluid is guided by perfusion and tolerance.Fluids complement adrenaline and must not delay repeat dosing. Monitor for overload and avoid a suspected causative colloid preparation.
08ComplicationsImportant consequences, why they occur and why they matter clinically.
Biphasic reaction
Symptoms may recur after initial resolution without another exposure, which is why observation duration and discharge preparedness depend on individual recurrence and severity risks.
Hypoxic or circulatory injury
Severe airway compromise and prolonged shock can injure the brain and myocardium even after the allergic process begins to respond to treatment.
Adrenaline administration error
Incorrect concentration, route or excessive intravenous dosing can cause severe hypertension and arrhythmia, especially when an arrest regimen is confused with anaphylaxis treatment.
09Monitoring and follow-upTreatment response, safety checks and longer-term review.
- Record the onset of symptoms and exact timing of every adrenaline dose, sampling event and documented clinical response.
- Continue cardiorespiratory observation during recovery and restart emergency assessment if airway swelling, wheeze, hypotension or collapse recurs.
- Choose the observation clock from resolution of the specified abnormalities, not merely from arrival or the first injection.
- Check practical injector use through demonstration and teach-back, and communicate the suspected reaction clearly to the patient and usual clinical team.
10Special situationsVariants, exceptions and circumstances that change the usual approach.
Supervised allergy challenge
NG258 allows specialist consideration of two-hour observation after resolution following a supervised allergy challenge even when two IM doses were required.
Children and tryptase
For children younger than sixteen, acute tryptase sampling is considered particularly for venom-related, drug-related or idiopathic reactions rather than routinely for every food reaction.
Beta-blocker exposure
Beta-blockade can complicate treatment response; persistent shock needs expert refractory-reaction management, including consideration of additional agents rather than repeated unmonitored IV boluses.
Pregnancy and anaphylaxis
Do not withhold indicated IM adrenaline during pregnancy. Support maternal oxygenation and circulation, relieve aortocaval compression when relevant and obtain obstetric assistance.
11Common pitfallsFrequent interpretation and management errors.
- 01
Do not wait for a rash, a tryptase result or confirmation of the allergen before treating ABC compromise.
- 02
Do not give an antihistamine in place of the first or repeated IM adrenaline dose.
- 03
Do not allow a recently hypotensive patient to stand suddenly because they say they feel better.
- 04
Do not apply obsolete universal observation rules or promise discharge before evaluating the NG258 risk factors.