Key messages
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Giving adults smaller amounts of red blood cells during a blood transfusion may not make any difference to their risk of dying, how long they stay in hospital, or any unwanted treatment effects, compared with giving larger amounts of red blood cells, but the evidence from some studies is unclear.
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At the same time, it may reduce the amount of blood given for one transfusion, which saves money and means that more blood is available for other people who need a transfusion.
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We did not find enough information to know whether it is better to give children or babies who need a transfusion larger or smaller amounts of red blood cells.
What is a blood transfusion?
Many people in hospital do not have enough healthy red blood cells to carry oxygen around their body, which can make them tired, weak, and short of breath. This is called anemia, and it can be treated in several different ways. Sometimes, people may need to have extra blood added to their own (a blood transfusion). Red blood cells are an important part of the extra blood. However, there is not much information available for doctors about the amount of red blood cells (the ‘red blood cell volume’) that should be given.
The volume of red blood cells in the blood can be measured in units (e.g. one unit of red blood cells) for adults. For babies and children, doctors give a certain amount depending on how much the child weighs. For example, they might give 20 mL of red blood cells for every kg the child weighs (20 mL/kg). Because blood transfusion involves some risks and there is a limited amount of blood available, doctors have been recommended to give the smallest effective amount to adults. Currently, it is recommended that one unit of red blood cells (rather than two units) is given to adults in hospital who are not bleeding and whose condition is not changing quickly. In children and newborns, there is no recommended standard amount for transfusion.
What did we want to find out?
We wanted to find out if it is safe to give a smaller amount of red blood cells to adults, children and newborns in hospital who need a blood transfusion. We looked at whether people stayed in hospital longer, had any unwanted effects from the treatment, any problems with their heart, breathing or other body systems, if they started bleeding again, and how many red blood cells were given.
What did we do?
We searched for studies that compared two different volumes of red blood cell transfusion for people in hospital. For example, two units of red blood cells compared with one unit of red blood cell for adults, or 20 mL/kg compared with 10 mL/kg for children. We compared and summarized the results of the studies and rated our confidence in the evidence, based on factors such as study methods and sizes.
What did we find?
We found 12 studies that involved 5478 people. Nine studies included a total of 1945 adults; one included women needing a blood transfusion after childbirth and the others included people who needed a blood transfusion because of blood cancer. One study included 3199 anemic children aged two months to 12 years, and two studies included a total of 334 newborn babies born before 32 weeks of pregnancy.
In adults, when comparing one unit of red blood cells to two units of red blood cells for a single blood transfusion, there may be no increased risk of dying, no increase in length of hospital stay, and no increase in any unwanted effects of the transfusion, blood clots (thrombosis) or starting to bleed again. However, the evidence is unclear.
What are the limitations of the evidence?
We have little confidence in most of the evidence we found because there were not enough studies to be certain about the results. In some of the studies, it is possible that people were aware of which treatment patients were getting. People in some of the studies were not randomly placed into the different treatment groups. This means that differences between the groups could be due to differences between people rather than between the treatments.
How up to date is this evidence?
The evidence is current to August 2025.
Read the full abstract
Objectives
To compare the effectiveness and safety of larger versus smaller RBC volume per transfusion for anemia in hospitalized adults, children, and preterm neonates.
Search strategy
We searched Evidence-Based Medicine Reviews (EBMR; including CENTRAL), MEDLINE, Embase, Web of Science, and other databases on 5 August 2025, with reference checking, citation searching and contacting study authors to identify additional studies.
Authors' conclusions
In adults, when comparing two units of RBCs (larger volume) to one unit of RBCs (smaller volume) for a single transfusion event, there was no difference in mortality, length of hospital stay or TAAEs, albeit with low or very low-certainty evidence. However, it reduced the number of required RBC units.
The results indicate that a larger transfusion volume confers no clinical benefit over a smaller, more restrictive transfusion strategy (lower transfusion volume), but may lead to a higher amount of blood administered. Given the adverse events associated with RBC transfusion and the resource limitation of the allogeneic blood supply, it is reasonable to support the recommendations for single unit RBC transfusion per transfusion event in adults.
In children, evidence is still too limited to be able to make a recommendation on an RBC volume.
Funding
This study did not receive funding.
Registration
This review protocol was previously published with the Cochrane database of systematic reviews (DOI 10.1002/14651858.CD015898).
