Fever HALO · Blood Transfusion Reactions Acronym

Published: Sep 15, 2025

Author: Emily Miao, MD, PharmD•

Editor: Alyssa Haag, MD•

Editor: Lily Guo, MD•

Editor: Kelsey LaFayette, DNP, ARNP, FNP-C•

Editor: Arianna Succi, MD

Illustrator: Jannat Day

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What is Fever HALO?

Fever HALO is an acronym that highlights the different types of adverse reactions that can occur following a blood transfusion with donated blood or blood products. However, thanks to advances in donor screening, automation of health systems, and improved treatment strategies, the risks and incidence of transfusion reactions have decreased.

Symptoms of blood transfusion reactions can be non-specific and may differ in severity, ranging from mild to life-threatening or fatal. Regardless of the type of transfusion reaction, the first step in management is to immediately stop the transfusion, obtain vital signs, and assess airway, breathing, and circulation. Timely recognition and management of blood transfusion reactions are important to improve patient care outcomes.

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What does “Fever” in Fever HALO mean?

“Fever” in Fever HALO refers to the fever observed in a febrile non-hemolytic transfusion reaction. Fever is typically low-grade, with a temperature elevation of 1 to 2 degrees Fahrenheit (F). This reaction is non-hemolytic in nature, meaning that hemolysis, or red blood cells destruction, does not occur. Febrile non-hemolytic transfusion reactions are thought to be caused by the accumulation of cytokines (i.e., immunogenic proteins released by the recipient’s immune cells) in blood products during storage. Symptoms accompanying the low-grade fever include chills, headache, and flushing. Because this is a non-hemolytic reaction, a hemolysis diagnostic work-up will result negative. Treatment consists of supportive care measures aimed at reducing symptoms (e.g., acetaminophen). The reaction should be documented in the individual’s electronic medical record, and prophylactic strategies to reduce future risks may include leukoreduction, which is the removal of white blood cells from cellular components of blood products before transfusion.

What does the “H” in Fever HALO mean?

The “H” in Fever HALO refers to hemolysis, which occurs in acute or delayed hemolytic transfusion reactions. Hemolytic transfusion reactions are caused by ABO incompatibility, which occurs when donor blood is incompatible with the recipient’s blood. ABO incompatibility can happen when a unit of blood is transfused to the wrong patient or is mislabeled during testing. The ABO group system determines which A, B, and O antigens are present on an individual’s red blood cells, and thus the antibodies they produce. For example, individuals with blood type A express A antigens on their red blood cell surface and produce antibodies against the B antigen; those with blood type AB express both A and B antigens and do not produce antibodies against either; and those with type O do not express either A or B antigens, thus produce antibodies against both. Therefore, the compatibility between the donor and recipient’s blood types is crucial in preventing hemolytic transfusion reactions. For example, a hemolytic transfusion reaction can occur if a recipient with type A blood, thus antibodies against the B antigen, receives blood from a type AB blood donor. In this case, the recipients anti-B antibodies recognize the B antigen on the donor red blood cells, leading to their destruction.

More specifically, hemolytic transfusion reactions can be subcategorized as acute, occurring within 15-20 minutes from the beginning of the transfusion; and delayed, developing more than 24 hours and up to 3 days after the transfusion. Acute hemolytic reactions may present with fever, chills, hypotension, back or flank pain, kidney failure, bleeding, and disseminated intravascular coagulation (DIC) (i.e., a life-threatening condition that results in abnormal blood clotting, platelet consumption, and bleeding). Treatment consists of immediate discontinuation of the transfusion, supportive care with intravenous fluids, and monitoring of urine output. Similarly, delayed hemolytic reactions may present with fever, anemia, jaundice, and malaise. Treatment is similar, consisting of stopping the transfusion, providing supportive care with intravenous fluids, and monitoring for the development of life-threatening complications.

What does the “A” in Fever HALO mean?

The “A” in Fever HALO refers to the allergic reaction observed in anaphylactic transfusion reactions due to hypersensitivity to one of the blood components. More specifically, anaphylaxis typically occurs due to an IgE-mediated hypersensitivity reaction in recipients lacking IgA or haptoglobin, a protein produced by the liver. Symptoms of anaphylactic transfusion reactions usually present within minutes and may include acute hypotension, angioedema (i.e., swelling under the skin), urticaria, and wheezing. Treatment consists of intramuscular epinephrine, corticosteroids (e.g., methylprednisolone), and diphenhydramine, along with supportive care for airway, breathing, and circulation as needed. Repeated assessments of the airway, breathing, circulation and vital signs should be performed to ensure hemodynamic stability.

What does the “L” in Fever HALO mean?

The “L” in Fever HALO refers to lung or lung injury, as seen in transfusion-related acute lung injury (TRALI). TRALI is characterized by the acute onset of non-cardiogenic pulmonary edema or lung injury typically occurring 1 to 6 hours after transfusion. While many hypotheses have been proposed to explain TRALI, it is most commonly attributed to the passive transfer of specific antibodies, such as human leukocyte antigen (HLA) or human neutrophil antigen (HNA) antibodies, from the donor’s plasma, which react with the recipient’s leukocyte antigens. The treatment for TRALI consists of supportive care with oxygen delivery and repeated assessments of airway, breathing, circulation, and vital signs. Intubation may be necessary if the airway is compromised or obstructed.

What does the “O” in Fever HALO mean?

The “O” in Fever HALO refers to the volume overload occurring in transfusion-associated circulatory overload (TACO). TACO happens when blood is transfused too quickly or in volumes exceeding the recipient’s circulatory capacity. Predisposing factors for circulatory overload and TACO include age, congestive heart failure, chronic kidney disease, and chronic anemia. Signs and symptoms typically occur within 1 to 6 hours after transfusion, and individuals typically experience cardiogenic edema (i.e., increased fluid retention and swelling secondary to an increased capillary hydrostatic pressure). Signs of cardiogenic edema include shortness of breath, difficulty breathing, hypoxemia, elevated jugular venous pressure, and hypertension. An elevated serum brain natriuretic peptide (BNP), which suggests volume overload, and a chest X-ray demonstrating pulmonary interstitial edema are diagnostic for TACO. Treatment consists of supportive care measures, such as supplemental oxygen delivery and intravenous diuretics to reduce fluid overload. To prevent future episodes of TACO, blood products should be transfused at a slower rate.

What are the most important facts to know about the Fever HALO acronym?

Fever HALO is an acronym that highlights the different types of adverse reactions that can occur following a blood transfusion with donated blood or blood products. Symptoms of blood transfusion reactions can be non-specific and may differ in severity, ranging from mild to life-threatening or fatal. The different types of transfusion reactions include febrile non-hemolytic transfusions (fever), acute or delayed hemolytic transfusion reactions (H), anaphylactic reactions (A), transfusion-related acute lung injury (L), and transfusion-associated circulatory overload (O). Regardless of the type of transfusion reaction, the first step in management is to stop the transfusion immediately, obtain vital signs, and assess the person's airway, breathing, and circulation.

Key Takeaways

Definition	Fever HALO is an acronym that highlights the different types of adverse reactions that can occur following a transfusion with donated blood or blood products.
Fever	Febrile non-hemolytic transfusion reaction
H	Hemolysis (acute or delayed hemolytic transfusion reactions)
A	Allergy (anaphylactic transfusion reactions)
L	Lung (transfusion-related acute lung injury, or TRALI)
O	Overload (transfusion-associated circulatory overload, or TACO)

References

Carman M, Uhlenbrock JS, McClintock SM. CE: A review of current practice in transfusion therapy. Am J Nurs. 2018;118(5):36-44. doi:10.1097/01.NAJ.0000532808.81713.fc

DeLisle J. Is this a blood transfusion reaction? Don't hesitate; Check it out. J Infus Nurs. 2018;41(1):43-51. doi:10.1097/NAN.0000000000000261

Garraud O, Cognasse F, Laradi S, et al. How to mitigate the risk of inducing transfusion-associated adverse reactions. Transfus Clin Biol. 2018;25(4):262-268. doi:10.1016/j.tracli.2018.07.006

Heddle NM. Pathophysiology of febrile nonhemolytic transfusion reactions. Curr Opin Hematol. 1999;6(6):420-426. doi:10.1097/00062752-199911000-00012

McClosky ME, Cimino Brown D, Weinstein NM, et al. Prevalence of naturally occurring non-AB blood type incompatibilities in cats and influence of crossmatch on transfusion outcomes. J Vet Intern Med. 2018;32(6):1934-1942. doi:10.1111/jvim.15334

Sirianni G, Perri G, Callum J, Gardner S, Berall A, Selby D. A retrospective chart review of transfusion practices in the palliative care unit setting. Am J Hosp Palliat Care. 2019;36(3):185-190. doi:10.1177/1049909118806456