Biphasic Anaphylaxis: Recognition and Risk Factors

anaphylaxis biphasic anaphylaxis Jun 01, 2026

Most perioperative anaphylaxis improves with timely treatment. But not all reactions follow a single, predictable course.

Biphasic anaphylaxis refers to the recurrence of symptoms after initial resolution, without further exposure to the trigger. This second phase can occur hours later and may range from mild to severe. In the perioperative setting, this creates uncertainty around recovery and discharge planning.

The underlying mechanism is not fully understood, but it is thought to involve a delayed release of inflammatory mediators after the initial response. Even when the patient appears stable, this process may still be evolving.

Recognising who is at risk helps guide observation.

  • ⚡ Severe initial reactions increase the likelihood of a biphasic response
    • ⚡ Delayed or inadequate adrenaline treatment is associated with higher risk
    • ⚡ Ongoing hypotension or airway involvement may signal a more complex course
    • ⚡ Requirement for repeated adrenaline doses suggests increased vulnerability
    • ⚡ Short observation periods may miss delayed recurrence

Observation is not one-size-fits-all.

In practice, patients with mild, rapidly resolving symptoms may require shorter monitoring, while those with severe or refractory features often need extended observation or higher-level care. Clinical judgement plays a central role, supported by local protocols.

For perianaesthesia nurses, this is where vigilance continues beyond initial stabilisation. Monitoring trends, reassessing symptoms, and preparing for potential recurrence are all part of safe care.

Discharge is not just about resolution. It is about confidence that the reaction has truly settled.

Understanding biphasic anaphylaxis helps ensure that patients are not only treated effectively, but also observed appropriately for what may come next.

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References:

 Kolawole, H., Marshall, S.D., Crilly, H., Kerridge, R., Roessler, P. (2023). Vasopressor selection in refractory anaphylaxis. Anaesthesia and Intensive Care.
Egner, W., Helbert, M., Sargur, R. (2021). Diagnostic approach to perioperative anaphylaxis and its mimics. Clinical and Experimental Allergy.

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