Transfusion Reactions and Adverse Events

A transfusion reaction is an adverse event associated with the transfusion of blood or a blood component. Reactions range from common and mild — such as febrile non-haemolytic or allergic reactions — to rare and life-threatening, including acute haemolytic reactions, transfusion-related acute lung injury (TRALI), and transfusion-associated circulatory overload (TACO). Recognising, classifying, and reporting these events is the work of haemovigilance and is central to transfusion safety.

Definition

Transfusion reactions are adverse reactions occurring during or after the administration of blood or blood components, classified by timing (acute versus delayed) and mechanism (immune versus non-immune) into recognised categories with consensus case definitions.

Scope

This topic surveys the principal categories of transfusion reaction — acute and delayed haemolytic, febrile non-haemolytic, allergic and anaphylactic, TRALI, TACO, and septic and other reactions — together with their broad mechanisms and the standardised case definitions used in surveillance. It describes how reactions are categorised and recognised; it is a reference and educational account and provides no diagnostic or treatment instructions for an individual patient.

Core questions

• Into what categories are transfusion reactions classified?
• What broad mechanisms distinguish haemolytic, lung-injury, and circulatory-overload reactions?
• How are TRALI and TACO defined for surveillance and how are they distinguished?
• How does haemovigilance use case definitions to monitor transfusion safety?

Key concepts

• Acute haemolytic transfusion reaction
• Delayed haemolytic transfusion reaction
• Febrile non-haemolytic transfusion reaction
• Allergic and anaphylactic reactions
• Transfusion-related acute lung injury (TRALI)
• Transfusion-associated circulatory overload (TACO)
• Transfusion-transmitted infection
• Haemovigilance and case definitions

Mechanisms

Reactions divide into immune and non-immune processes. Acute haemolytic reactions, most often from ABO-incompatible transfusion, occur when recipient antibodies bind donor red-cell antigens and activate complement, causing intravascular haemolysis; delayed haemolytic reactions reflect an anamnestic antibody response days to weeks later, with predominantly extravascular red-cell destruction (Panch et al., 2019). TRALI is an acute, non-cardiogenic pulmonary oedema attributed to a 'two-hit' interaction of recipient predisposition with donor leucocyte antibodies or biologically active mediators, and its diagnosis follows a consensus case definition (Vlaar et al., 2019). TACO, by contrast, is hydrostatic pulmonary oedema from volume excess and is defined by a separate surveillance case definition that emphasises features of circulatory overload (Wiersum-Osselton et al., 2019). Febrile and allergic reactions arise from cytokines and from recipient antibodies to plasma proteins, respectively.

Clinical relevance

Transfusion reactions are the principal harms weighed against the benefit of transfusion, and standardised case definitions allow them to be detected, reported, and reduced through haemovigilance and donor-management measures. This entry explains how reactions are categorised and recognised at a conceptual level; it is not a clinical protocol and does not provide instructions for diagnosing or managing a reaction in a specific patient, which require trained clinicians and local guidelines (Carson et al., 2017).

Epidemiology

Mild reactions such as febrile non-haemolytic and allergic events are relatively common, whereas fatal acute haemolytic reactions are rare and largely preventable, typically resulting from clerical or identification errors leading to ABO-incompatible transfusion. TRALI and TACO have been recognised as leading causes of transfusion-related morbidity and mortality in haemovigilance reports, prompting the revised consensus definitions that now standardise their surveillance (Panch et al., 2019; Vlaar et al., 2019; Wiersum-Osselton et al., 2019).

History

As transfusion became routine, haemolytic reactions from blood-group incompatibility were the first recognised hazard, and the identification of ABO incompatibility as the dominant cause of fatal reactions drove the patient-identification safeguards that define modern practice. Later, two pulmonary syndromes were disentangled: TRALI was characterised as an immune or mediator-driven acute lung injury, and TACO as circulatory overload, each receiving an internationally agreed case definition in 2019 to harmonise reporting (Vlaar et al., 2019; Wiersum-Osselton et al., 2019).

Debates

How should TRALI be distinguished from TACO at the bedside and in surveillance?

Both present as acute pulmonary oedema after transfusion but differ in mechanism — immune or mediator-driven lung injury versus hydrostatic volume overload — and they can overlap. The 2019 consensus redefinitions sharpened the criteria, yet reliable separation in individual cases and in haemovigilance data remains challenging.

Key figures

• Harvey Klein
• Alexander Vlaar
• Pearl Toy
• Steven Kleinman

Seminal works

• panch-2019
• vlaar-2019
• wiersum-osselton-2019

Frequently asked questions

What is the most dangerous type of transfusion reaction?

Acute haemolytic transfusion reactions, usually caused by ABO-incompatible blood given after an identification error, can be rapidly fatal. They are rare because of patient-identification safeguards, and TRALI and TACO are also recognised as leading causes of serious transfusion-related harm.

How do TRALI and TACO differ?

Both cause acute pulmonary oedema after transfusion, but TRALI is a non-cardiogenic, immune or mediator-driven lung injury, whereas TACO is cardiogenic pulmonary oedema from circulatory volume overload. Each has its own consensus surveillance case definition.