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. 2021 Dec 21;12(12):CD002042.
doi: 10.1002/14651858.CD002042.pub5.

Transfusion thresholds for guiding red blood cell transfusion

Jeffrey L Carson  1 , Simon J Stanworth  2   3   4 , Jane A Dennis  5 , Marialena Trivella  6 , Nareg Roubinian  7 , Dean A Fergusson  8 , Darrell Triulzi  9 , Carolyn Dorée  4 , Paul C Hébert  10
Affiliations

Transfusion thresholds for guiding red blood cell transfusion

Jeffrey L Carson et al. Cochrane Database Syst Rev. .

Abstract

Background: The optimal haemoglobin threshold for use of red blood cell (RBC) transfusions in anaemic patients remains an active field of research. Blood is a scarce resource, and in some countries, transfusions are less safe than in others because of inadequate testing for viral pathogens. If a liberal transfusion policy does not improve clinical outcomes, or if it is equivalent, then adopting a more restrictive approach could be recognised as the standard of care. OBJECTIVES: The aim of this review update was to compare 30-day mortality and other clinical outcomes for participants randomised to restrictive versus liberal red blood cell (RBC) transfusion thresholds (triggers) for all clinical conditions. The restrictive transfusion threshold uses a lower haemoglobin concentration as a threshold for transfusion (most commonly, 7.0 g/dL to 8.0 g/dL), and the liberal transfusion threshold uses a higher haemoglobin concentration as a threshold for transfusion (most commonly, 9.0 g/dL to 10.0 g/dL).

Search methods: We identified trials through updated searches: CENTRAL (2020, Issue 11), MEDLINE (1946 to November 2020), Embase (1974 to November 2020), Transfusion Evidence Library (1950 to November 2020), Web of Science Conference Proceedings Citation Index (1990 to November 2020), and trial registries (November 2020). We checked the reference lists of other published reviews and relevant papers to identify additional trials. We were aware of one trial identified in earlier searching that was in the process of being published (in February 2021), and we were able to include it before this review was finalised.

Selection criteria: We included randomised trials of surgical or medical participants that recruited adults or children, or both. We excluded studies that focused on neonates. Eligible trials assigned intervention groups on the basis of different transfusion schedules or thresholds or 'triggers'. These thresholds would be defined by a haemoglobin (Hb) or haematocrit (Hct) concentration below which an RBC transfusion would be administered; the haemoglobin concentration remains the most commonly applied marker of the need for RBC transfusion in clinical practice. We included trials in which investigators had allocated participants to higher thresholds or more liberal transfusion strategies compared to more restrictive ones, which might include no transfusion. As in previous versions of this review, we did not exclude unregistered trials published after 2010 (as per the policy of the Cochrane Injuries Group, 2015), however, we did conduct analyses to consider the differential impact of results of trials for which prospective registration could not be confirmed. DATA COLLECTION AND ANALYSIS: We identified trials for inclusion and extracted data using Cochrane methods. We pooled risk ratios of clinical outcomes across trials using a random-effects model. Two review authors independently extracted data and assessed risk of bias. We conducted predefined analyses by clinical subgroups. We defined participants randomly allocated to the lower transfusion threshold as being in the 'restrictive transfusion' group and those randomly allocated to the higher transfusion threshold as being in the 'liberal transfusion' group.

Main results: A total of 48 trials, involving data from 21,433 participants (at baseline), across a range of clinical contexts (e.g. orthopaedic, cardiac, or vascular surgery; critical care; acute blood loss (including gastrointestinal bleeding); acute coronary syndrome; cancer; leukaemia; haematological malignancies), met the eligibility criteria. The haemoglobin concentration used to define the restrictive transfusion group in most trials (36) was between 7.0 g/dL and 8.0 g/dL. Most trials included only adults; three trials focused on children. The included studies were generally at low risk of bias for key domains including allocation concealment and incomplete outcome data. Restrictive transfusion strategies reduced the risk of receiving at least one RBC transfusion by 41% across a broad range of clinical contexts (risk ratio (RR) 0.59, 95% confidence interval (CI) 0.53 to 0.66; 42 studies, 20,057 participants; high-quality evidence), with a large amount of heterogeneity between trials (I² = 96%). Overall, restrictive transfusion strategies did not increase or decrease the risk of 30-day mortality compared with liberal transfusion strategies (RR 0.99, 95% CI 0.86 to 1.15; 31 studies, 16,729 participants; I² = 30%; moderate-quality evidence) or any of the other outcomes assessed (i.e. cardiac events (low-quality evidence), myocardial infarction, stroke, thromboembolism (all high-quality evidence)). High-quality evidence shows that the liberal transfusion threshold did not affect the risk of infection (pneumonia, wound infection, or bacteraemia). Transfusion-specific reactions are uncommon and were inconsistently reported within trials. We noted less certainty in the strength of evidence to support the safety of restrictive transfusion thresholds for the following predefined clinical subgroups: myocardial infarction, vascular surgery, haematological malignancies, and chronic bone-marrow disorders.

Authors' conclusions: Transfusion at a restrictive haemoglobin concentration decreased the proportion of people exposed to RBC transfusion by 41% across a broad range of clinical contexts. Across all trials, no evidence suggests that a restrictive transfusion strategy impacted 30-day mortality, mortality at other time points, or morbidity (i.e. cardiac events, myocardial infarction, stroke, pneumonia, thromboembolism, infection) compared with a liberal transfusion strategy. Despite including 17 more randomised trials (and 8846 participants), data remain insufficient to inform the safety of transfusion policies in important and selected clinical contexts, such as myocardial infarction, chronic cardiovascular disease, neurological injury or traumatic brain injury, stroke, thrombocytopenia, and cancer or haematological malignancies, including chronic bone marrow failure. Further work is needed to improve our understanding of outcomes other than mortality. Most trials compared only two separate thresholds for haemoglobin concentration, which may not identify the actual optimal threshold for transfusion in a particular patient. Haemoglobin concentration may not be the most informative marker of the need for transfusion in individual patients with different degrees of physiological adaptation to anaemia. Notwithstanding these issues, overall findings provide good evidence that transfusions with allogeneic RBCs can be avoided in most patients with haemoglobin thresholds between the range of 7.0 g/dL and 8.0 g/dL. Some patient subgroups might benefit from RBCs to maintain higher haemoglobin concentrations; research efforts should focus on these clinical contexts.

Trial registration: ClinicalTrials.gov NCT01648946 NCT00071032 NCT01167582 NCT00126334 NCT01502215 NCT02086773 NCT02648113 NCT00162617 NCT03407573 NCT02203292 NCT01102010 NCT01021631 NCT01485315 NCT00651573 NCT02042898 NCT02465125 NCT00906295 NCT01079247 NCT00335023 NCT00470444 NCT01237639 NCT00414713 NCT00944112 NCT01116479 NCT01393496 NCT03419780 NCT02099669 NCT03369210 NCT02619136 NCT02981407 NCT03229941 NCT02483351 NCT03309579 NCT03871244 NCT04506125 NCT04754022.

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Conflict of interest statement

We have considered disclosures relevant to this review.

Jeffrey Carson reports being the chief investigator on three trials included in this review (Carson 1998; Carson 2011 (FOCUS); Carson 2013). He has received multiple grants supporting his institution from the US National Institutes of Health. He has been involved with guideline development for red cell transfusions in the Association for the Advancement of Blood & Biotherapies (AABB). He has received a grant from the US National Institutes of Health to evaluate transfusion thresholds in patients with acute myocardial infarction in an ongoing trial (MINT ‐ NCT02981407).

Carolyn Dorée: nothing to declare.

Dean Fergusson reports being an author on three completed trials included within this review (Hébert 1999; Mazer 2017 (TRICS III); Shehata 2012). He was a Co‐Principal Investigator on the TRICSIII trial (Mazer 2017); he is a member of the Steering Committee for the ongoing MINT trial (NCT02981407).

Paul Hébert reports being an author on three completed trials identified in this review (Hébert 1995; Hébert 1999; Lacroix 2007 (TRIPICU). He is the lead investigator on the study NCT02619136, the Canadian pilot study for MINT (NCT02981407, for which he is a member of the Executive Committee); he has published six non‐Cochrane reviews in this area. He serves on a guideline panel for the American College of Chest Physicians (ACCP) and has written several editorials for leading journals concerning transfusion triggers.

Nareg Roubinian: nothing to declare.

Simon Stanworth reports being the chief investigator on one trial included in this review (Stanworth 2020); a coauthor on another (Gillies 2020 (RESULT‐NOF)); and is a co‐investigator on four ongoing trials (ISRCTN17438123; ACTRN12619001053112; NCT03871244; Morton 2020). He has received funding for four RBC transfusion trials, including three in patients with haematological malignancy. He has published three non‐Cochrane reviews in this area.

Darrell Triulzi is a member of the Steering Committee for the ongoing MINT trial (NCT02981407) and a member of the scientific advisory board for Fresenius‐Kabi.

Jane Dennis was employed by Cochrane Injuries during her involvement in development of the review.

Marialena Trivella was employed by Cochrane Injuries during her involvement in development of the review.

Data extraction for all trials was checked by NR. Final decisions on risk of bias assessments were made by review authors not involved in trials as researchers.

Figures

1
1
Flow of studies for 2021 update
2
2
'Risk of bias' graph: review authors' judgements about each 'Risk of bias' item presented as percentages across all included trials. Forty‐eight trials are included in this review.
3
3
'Risk of bias' summary: review authors' judgements about each methodological quality item for each included trial
4
4
Funnel plot of comparison: 1 Mortality, outcome: 1.1 30‐Day mortality
5
5
Funnel plot of comparison: 2 Blood transfusions, outcome: 2.1 Participants exposed to blood transfusion (all trials)
6
6
Funnel plot of comparison: 2 Subgroup analysis by prospective registration, outcome: 2.1 30‐Day mortality
1.1
1.1. Analysis
Comparison 1: Mortality at 30 days, Outcome 1: 30‐Day mortality
1.2
1.2. Analysis
Comparison 1: Mortality at 30 days, Outcome 2: 30‐Day mortality subgroup by restrictive haemoglobin level
1.3
1.3. Analysis
Comparison 1: Mortality at 30 days, Outcome 3: 30‐Day mortality subgroup analysis by clinical specialties
1.4
1.4. Analysis
Comparison 1: Mortality at 30 days, Outcome 4: 30‐Day mortality by clinical specialties: myocardial infarction vs all others
1.5
1.5. Analysis
Comparison 1: Mortality at 30 days, Outcome 5: Mortality by cardiac surgery, vascular surgery, myocardial infarction, and all others
2.1
2.1. Analysis
Comparison 2: Subgroup analysis by prospective registration, Outcome 1: 30‐Day mortality
3.1
3.1. Analysis
Comparison 3: Sensitivity analysis by allocation concealment, Outcome 1: 30‐Day mortality
4.1
4.1. Analysis
Comparison 4: Mortality: other time intervals, Outcome 1: Hospital mortality
4.2
4.2. Analysis
Comparison 4: Mortality: other time intervals, Outcome 2: 90‐Day mortality
4.3
4.3. Analysis
Comparison 4: Mortality: other time intervals, Outcome 3: 6‐Month mortality
5.1
5.1. Analysis
Comparison 5: Morbidity: clinical outcomes, Outcome 1: Cardiac events
5.2
5.2. Analysis
Comparison 5: Morbidity: clinical outcomes, Outcome 2: Myocardial infarction
5.3
5.3. Analysis
Comparison 5: Morbidity: clinical outcomes, Outcome 3: Congestive heart failure
5.4
5.4. Analysis
Comparison 5: Morbidity: clinical outcomes, Outcome 4: Cerebrovascular accident (CVA) ‐ stroke
5.5
5.5. Analysis
Comparison 5: Morbidity: clinical outcomes, Outcome 5: Rebleeding
5.6
5.6. Analysis
Comparison 5: Morbidity: clinical outcomes, Outcome 6: Sepsis/bacteraemia
5.7
5.7. Analysis
Comparison 5: Morbidity: clinical outcomes, Outcome 7: Pneumonia
5.8
5.8. Analysis
Comparison 5: Morbidity: clinical outcomes, Outcome 8: Infection
5.9
5.9. Analysis
Comparison 5: Morbidity: clinical outcomes, Outcome 9: Thromboembolism
5.10
5.10. Analysis
Comparison 5: Morbidity: clinical outcomes, Outcome 10: Renal failure
5.11
5.11. Analysis
Comparison 5: Morbidity: clinical outcomes, Outcome 11: Mental confusion
6.1
6.1. Analysis
Comparison 6: Blood transfusions, Outcome 1: Participants exposed to blood transfusion (all trials)
6.2
6.2. Analysis
Comparison 6: Blood transfusions, Outcome 2: Participants exposed to blood transfusion by clinical specialties
6.3
6.3. Analysis
Comparison 6: Blood transfusions, Outcome 3: Participants exposed to blood transfusion (by transfusion threshold)
6.4
6.4. Analysis
Comparison 6: Blood transfusions, Outcome 4: Participants exposed to blood transfusion by transfusion threshold
6.5
6.5. Analysis
Comparison 6: Blood transfusions, Outcome 5: Units of blood transfused
7.1
7.1. Analysis
Comparison 7: Morbidity outcomes in participants undergoing cardiac surgery or vascular surgery, and with acute MI, Outcome 1: Myocardial infarction
7.2
7.2. Analysis
Comparison 7: Morbidity outcomes in participants undergoing cardiac surgery or vascular surgery, and with acute MI, Outcome 2: Renal failure
7.3
7.3. Analysis
Comparison 7: Morbidity outcomes in participants undergoing cardiac surgery or vascular surgery, and with acute MI, Outcome 3: Infection
7.4
7.4. Analysis
Comparison 7: Morbidity outcomes in participants undergoing cardiac surgery or vascular surgery, and with acute MI, Outcome 4: Congestive heart failure
7.5
7.5. Analysis
Comparison 7: Morbidity outcomes in participants undergoing cardiac surgery or vascular surgery, and with acute MI, Outcome 5: Thromboembolism
7.6
7.6. Analysis
Comparison 7: Morbidity outcomes in participants undergoing cardiac surgery or vascular surgery, and with acute MI, Outcome 6: Cerebrovascular accident
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Update of

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Palmieri 2017 {published data only}
    1. Palmieri TL, Holmes JH, Arnoldo B, Peck M, Cochran A, King BT, et al.Restrictive transfusion strategy is more effective in massive burns: results of the TRIBE multicenter prospective randomized trial. Military Medicine 2019;184 (Suppl 1):11-5. [DOI: doi: 10.1093/milmed/usy279] - PMC - PubMed
    1. Palmieri TL, Holmes JH, Arnoldo B, Peck M, Potenza B, Cochran A, et al.Transfusion requirement in burn care evaluation (TRIBE). A multicenter randomized prospective trial of blood transfusion in major burn injury. Annals of Surgery 2017;266:595-602. - PMC - PubMed
Parker 2013 {published data only}61328173
    1. Parker MJ.Randomised trial of blood transfusion versus a restrictive transfusion policy after hip fracture surgery. Injury 2013;44(12):1916-8. - PubMed
Prick 2014 {published data only}
    1. Prick BW, Duvekot JJ, Moer PE, Gemund N, Salm PC, Jansen AJ, et al.Cost-effectiveness of red blood cell transfusion vs. non-intervention in women with acute anaemia after postpartum haemorrhage. Vox Sanguinis 2014;107(4):381-8. [DOI: 10.1111/vox.12181] - DOI - PubMed
    1. Prick BW, Jansen AJ, Steegers EA, Hop WC, Essink-Bot ML, Uyl-de Groot CA, et al.Transfusion policy after severe postpartum haemorrhage: a randomised non-inferiority trial. BJOG 2014;121(8):1005-14. - PubMed
Robitaille 2013 {published data only}
    1. Robitaille N, Lacroix J, Alexandrov L, Clayton L, Cortier M, Schultz KR, et al.Excess of veno-occlusive disease in a randomized trial on a higher trigger for red cell transfusion after bone marrow transplantation: a Canadian blood and marrow transplant group trial. Biology of Blood and Marrow Transplantation 2013;19:468-73. - PubMed
Shehata 2012 {published data only}
    1. Shehata N, Burns LA, Nathan H, Hebert P, Hare GM, Fergusson D, et al.A randomized controlled pilot study of adherence to transfusion strategies in cardiac surgery. Transfusion 2012;52(1):91-9. - PubMed
So‐Osman 2013 {published data only}
    1. So-Osman C, Nelissen R, Brand R, Faber F, Slaa RT, Stiggelbout A, et al.The impact of a restrictive transfusion trigger on post-operative complication rate and well-being following elective orthopaedic surgery: a post-hoc analysis of a randomised study. Blood Transfusion (Trasfusione del sangue) 2013;11(2):289-95. - PMC - PubMed
    1. So-Osman C, Nelissen R, Te Slaa R, Coene L, Brand R, Brand A.A randomized comparison of transfusion triggers in elective orthopaedic surgery using leucocyte-depleted red blood cells. Vox Sanguinis 2010;98(1):56-64. - PubMed
    1. So-Osman C.A restrictive transfusion trigger is a method for blood saving in elective orthopaedic surgery. Vox Sanguinis 2004;87(Suppl 3):52.
Stanworth 2020 {published data only}26088319
    1. Stanworth SJ, Killick S, McQuilten ZK, Karakantza M, Weinkove R, Smethurst H, et al, REDDS Investigators.Red cell transfusion in outpatients with myelodysplastic syndromes: a feasibility and exploratory randomised trial. British Journal of Haematology 2020;189(2):279-90. [ISRCTN26088319] - PubMed
Tay 2020 {published data only}
    1. Tay J, Allan DS, Chatelain E, Coyle D, Elemary M, Fulford A, et al.Liberal versus restrictive red blood cell transfusion thresholds in hematopoietic cell transplantation: a randomized, open label, phase III, noninferiority trial. Journal of Clinical Oncology 2020;38(13):1463-73. - PubMed
Topley 1956 {published data only}
    1. Fisher MR, Topley ET.The illness of trauma. British Journal of Clinical Practice 1956;10(11):770-6. - PubMed
Villanueva 2013 {published and unpublished data}
    1. Colomo A, Hernandez-Gea V, Muniz-Diaz E, Madoz P, Aracil C, Alvarez-Urturi C.Transfusion strategies in patients with cirrhosis and acute gastrointestinal bleeding. Hepatology 2008;48(4 Suppl):413A.
    1. Villanueva C, Colomo A, Bosch A, Concepción M, Hernandez-Gea V, Aracil C, et al.Transfusion strategies for acute upper gastrointestinal bleeding. New England Journal of Medicine 2013;368(1):11-21. - PubMed
Walsh 2013 {published data only}
    1. Walsh TS, Boyd JA, Watson D, Hope D, Lewis S, Krishan A, et al.Restrictive versus liberal transfusion strategies for older mechanically ventilated critically ill patients: a randomized pilot trial. Critical Care Medicine 2013;41(10):2354-63. - PubMed
Webert 2008 {published and unpublished data}
    1. Webert KE, Cook RJ, Couban S, Carruthers J, Lee KA, Blajchman MA, et al.A multicenter pilot-randomized controlled trial of the feasibility of an augmented red blood cell transfusion strategy for patients treated with induction chemotherapy for acute leukemia or stem cell transplantation. Transfusion 2008;48(1):81-91. - PubMed
Yakymenko 2018 {published data only}
    1. Yakymenko D, Frandsen KB, Christensen IJ, Norgaard A, Johansson PI, Daugaard G, et al.Randomised feasibility study of a more liberal haemoglobin trigger for red blood cell transfusion compared to standard practice in anaemic cancer patients treated with chemotherapy. Transfusion Medicine 2018;28(3):208-15. - PubMed

References to studies excluded from this review

Atilla 2017 {published data only}
    1. Atilla E, Toprak SK, Civriz Bozdag S, Topcuoglu P, Arslan O.A randomized comparison of hemoglobin content-based versus standard (unit-based) red blood cell transfusion policy. Turkish Journal of Haematology 2017;34(244):244-9. - PMC - PubMed
Baron‐Stefaniak 2019 {published data only}
    1. Baron-Stefaniak J, Leitner GC, Kuntzel NKI, Meyer EL, Hiesmayr MJ, Ullrich R, et al.Transfusion of standard-issue packed red blood cells induces pulmonary vasoconstriction in critically ill patients after cardiac surgery - a randomized,double-blinded, clinical trial. PLOS One 2019;14(3):e0213000. - PMC - PubMed
Cholette 2017 {published data only}
    1. Cholette JM, Swartz MF, Rubenstein J, Henrichs KF, Wang H, Powers KS.Outcomes using a conservative versus liberal red blood cell transfusion strategy in infants requiring cardiac operation. Annals of Thoracic Surgery 2017;103(1):206-14. - PubMed
de Bruin 2019 {published data only}
    1. Bruin S, Scheeren TW, Bakker J, Bruggen R, Vlaar AP, Cardiovascular Dynamics Section and Transfusion Guideline Task Force of theESICM.Cardiovascular Dynamics Section and Transfusion Guideline Task Force of the ESICM. Transfusion practice in the non-bleeding critically ill: an international online survey - the TRACE survey. Critical Care 2019;23:309. [DOI: 10.1186/s13054-019-2591-6] - DOI - PMC - PubMed
Elshinawy 2020 {published data only}
    1. Elshinawy M, Al Marhoobi N, Al Abri R, Nazir HF, Khater D, Maktoom M, et al.Preoperative transfusion versus no transfusion policy in sickle cell disease patients: a randomized trial. Transfusion 2020;60(Suppl 1):S22-7. - PubMed
Fogagnolo 2020 {published data only}
    1. Fogagnolo A, Taccone FS, Vincent JL, Benetto G, Cavalcante E, Marangoni E, et al.Using arterial-venous oxygen difference to guide red blood cell transfusion strategy. Critical Care 2020;24:160. - PMC - PubMed
Fortune 1987 {published data only}
    1. Fortune JB, Feustel PJ, Saifi J, Stratton HH, Newell JC, Shah DM.Influence of hematocrit on cardiopulmonary function after acute hemorrhage. Journal of Trauma 1987;27(3):243-9. - PubMed
Franz 2020 {published data only}
    1. Franz AR, Engel C, Bassler D, Rüdiger M, Thome UH, Maier RF, et al, the ETTNO Investigators.Effects of liberal vs restrictive transfusion thresholds on survival and neurocognitive outcomes in extremely low-birth-weight infants: the ETTNO randomized clinical trial. JAMA 2020;324:560-70. [CTG: ] [EUCTR: EUCTR2010-021576-28-DE] - PMC - PubMed
Haensig 2019 {published data only}
    1. Haensig M, Kempfert J, Kempfert PM, Girdauskas E, Borger MA, Lehmann S.Thrombelastometry guided blood-component therapy after cardiac surgery: a randomized study. BMC Anesthesiology 2019;19(1):201. [DOI: 10.1186/s12871-019-0875-7] - DOI - PMC - PubMed
Hamm 2020 {published data only}
    1. Hamm RF, Perelman S, Wang EY, Levine LD, Srinivas SK.Single-unit vs multiple-unit transfusion in hemodynamically stable postpartum anemia: a pragmatic randomized controlled trial. American Journal of Obstetrics & Gynecology 2021;224(84):e1-7.0002-9378/. [CTG: ] - PubMed
Jain 2019 {published data only}
    1. Jain A, Raja A, Marwaha N, Trehan A.Comparison of efficacy of packed red blood cell transfusion based on its hemoglobin content versus the routine transfusion practice in thalassemia major patients. Vox Sanguinis 2019;114:213. - PubMed
    1. Raja A, Jain A, Marwaha N, Trehan A.Comparison of efficacy of packed red blood cell transfusion based on its hemoglobin content versus the standard transfusion practice in thalassemia major patients (HEMOCON study). Transfusion and Apheresis Science 2020;59:6 (Article ID: 102736). [DOI: ] - PubMed
Kirpalani 2020 {published data only}
    1. Kirpalani H, Bell EF, Hintz SR, Tan S, Schmidt B, Chaudhary AS, et al .Higher or lower hemoglobin transfusion thresholds for preterm infants . New England Journal of Medicine 2020;383(27):2639-51. - PMC - PubMed
Koster 2016 {published data only}
    1. Koster A, Zittermann A, Borgermann J, Knabbe C, Diekmann J, Schirmer U, et al.Transfusion of 1 and 2 units of red blood cells does not increase mortality and organ failure in patients undergoing isolated coronary artery bypass grafting. European Journal of Cardio-thoracic Surgery 2016;49:931-6. - PubMed
Kumar 2019 {published data only}
    1. Kumar M, Ahmad J, Maiwall R, Choudhury A, Bajpai M, Mitra LG, et al.Thromboelastography-guided blood component use in patients with cirrhosis with nonvariceal bleeding: a randomized controlled trial. Hepatology 2019;71(1):235-46. - PubMed
Leal‐Noval 2017 {published data only}
    1. Leal-Noval SR, Arellano-Orden V, Maestre-Romero A, Muñoz-Gómez M, Fernández-Cisneros V, Ferrándiz-Millón C, et al.Red blood cell transfusion guided by near nfrared spectroscopy in neurocritically ill patients with moderate or severe anemia: a randomized, controlled trial. Journal of Neurotrauma 2017;34(17):2553-9. - PubMed
Osawa 2016 {published data only}
    1. Osawa EA, Rhodes A, Landoni G, Galas FR, Fukushima JT, Park CH, et al.Effect of perioperative goal-directed hemodynamic resuscitation therapy on outcomes following cardiac surgery: a randomized clinical trial and systematic review. Critical Care Medicine 2016;44(4):724-33. [DOI: 10.1097/CCM.0000000000001479] - DOI - PubMed
Robertson 2014 {published data only}
    1. Robertson CS, Hannay HJ, Yamal JM, Gopinath S, Goodman JC, Tilley BC, et al.Effect of erythropoietin and transfusion threshold on neurological recovery after traumatic brain injury: a randomized clinical trial. JAMA 2014;312(1):36-47. - PMC - PubMed
Vichinsky 1995 {published data only}
    1. Vichinsky EP, Haberkern CM, Neumayr L, Earles AN, Black D, Koshy M, et al.A comparison of conservative and aggressive transfusion regimens in the perioperative management of sickle cell disease. The Preoperative Transfusion in Sickle Cell Disease Study Group. New England Journal of Medicine 1995;333(4):206-13. - PubMed
Voorn 2017 {published data only}
    1. Voorn VM, Marang-van de Mheen PJ, Hout A, Hofstede SN, So-Osman C, den Akker-van Marle ME, et al.The effectiveness of a de-implementation strategy to reduce low-value blood management techniques in primary hip and knee arthroplasty: a pragmatic cluster-randomized controlled trial. Implementation Science 2017;12(1):72. [DOI: 10.1186/s13012-017-0601-0] - DOI - PMC - PubMed
Yamada 2020 {published data only}
    1. Yamada C, Takeshita A, Ohto H, Ishimaru K, Kawabata K, Nomaguchi Y, et al.A Japanese multi-institutional collaborative study of antigen-positive red blood cell (RBC) transfusions in patients with corresponding RBC antibodies. Vox Sanguinis 2020;115(5):456-65. - PubMed
Zygun 2009 {published data only}
    1. Zygun DA, Nortje J, Hutchinson PJ, Timofeev I, Menon DK, Gupta AK.The effect of red blood cell transfusion on cerebral oxygenation and metabolism after severe traumatic brain injury. Critical Care Medicine 2009;37(3):1074-8. - PubMed

References to studies awaiting assessment

IRCT20190209042660N1 {unpublished data only}20190209042660N1
    1. IRCT20190209042660N1.A clinical trial on comparing morbidity and mortality of restrictive blood transfusion strategy with a traditional strategy in patients with a thermal burn. Iranian Registry of Clinical Trials - Beta version. https://en.irct.ir/trial/43402 (first posted 15 June 2020).
ISRCTN26088319 {unpublished data only}26088319
    1. ISRCTN26088319.Red cell transfusion and QoL in myelodysplastic syndromes [Red blood cell transfusion thresholds and quality of life in myelodysplastic syndromes: a pilot, feasibility study]. www.isrctn.com/ISRCTN26088319 (first received 13 November 2014).
Maitland 2019 {published data only}
    1. Maitland K, Kiguli S, Olupot-Olupot, Engoru C, Mallewa M, Saramago Goncalves P, et al, the TRACT Group.Immediate transfusion in African children with uncomplicated severe anaemia. New England Journal of Medicine 2019;381(5):407-19. [DOI: 10.1056/NEJMoa1900105] [ISRCTN: ISRCTN84086586] - DOI - PMC - PubMed
    1. Maitland K, Polupot-Olupot P, Kiguli S, Chagaluka G, Alaroker F, Opoka RO, et al, TRACT Group.Transfusion volume for children with severe anemia in Africa. New England Journal of Medicine 2019;381(5):420-31. [DOI: 10.1056/NEJMoa1900100] - DOI - PMC - PubMed
Morton 2020 {published data only}
    1. Morton S, Sekhar M, Smethurst H, Mora S, Hodge R, Hudson C, et al.Results for the REAL pilot randomised trial of red cell transfusion in acute myeloid leukaemia: is there sufficient evidence of equipoise to support a definitive study? British Journal of Haematology 2020;189 (Suppl 1):36-7. [ISRCTN: ISRCTN96390716 ]
NCT02099669 {unpublished data only}
    1. Buckstein RJ, Prica A, Leber B, Heddle N, Yee KW, Stanworth SJ, et al.RBC-Enhance: a randomized pilot feasibility trial of red cell transfusion thresholds in myelodysplastic syndromes. Blood 2020;136(Suppl 1):3-4. [CTG: ]
    1. NCT02099669.Red blood cell transfusion thresholds and QOL in MDS (EnhanceRBC): a pilot, feasibility study. www.clinicaltrials.gov/ct2/show/NCT02099669 2014 (last accessed 6/7/21). [CTG: ]

References to ongoing studies

ACTRN12619001053112 {published data only}
    1. ACTRN12619001053112.Red blood cell transfusion schedule in myelodysplastic syndromes: study 2 (REDDS-2) [A randomised feasibility n-of-1 trial of weekly-interval red cell transfusion myelodysplastic syndromes]. www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=377401&isRe... (first received 4 June 2019). [ACTRN: ACTRN12619001053112]
Hayakawa 2020 {published data only}
    1. Hayakawa M, Tagami T, IIjima H, Kudo D, Sekine K, Ogura T, et al.Restrictive transfusion strategy for critically injured patients (RESTRIC) trial: a study protocol for a cluster-randomised, crossover non-inferiority trial. BMJ Open 2020 2020;10(9):e037238. [UMIN000034405] - PMC - PubMed
ISRCTN17438123 {unpublished data only}
    1. ISRCTN17438123.A small pilot feasibility study for a possible randomised control trial comparing clinical outcomes and quality of life following two different transfusion strategies in children undergoing allogeneic hematopoietic stem cell transplant (HSCT) [RePAST: Red cell transfusion in Paediatric Allogeneic HSCT - A feasibility randomised controlled trial comparing restrictive versus liberal red cell (RBC) transfusion strategies in children undergoing allogeneic haematopoietic stem cell transplant (HSCT)]. www.isrctn.com/ISRCTN17438123 (first posted 23 May 2019). [ISRCTN: ISRCTN17438123]
Meybohm 2019 {published data only}
    1. Meybohm P, Lindau S, Treskatsch S, Francis R, Spies C, Velten M, et al, LIBERAL Collaboration Group.Liberal transfusion strategy to prevent mortality and anaemia-associated, ischaemic events in elderly non-cardiac surgical patients - The study design of the LIBERAL-Trial. Trials 2019;20(1):101. [CTG: ] [DOI: 10.1186/s13063-019-3200-3] - DOI - PMC - PubMed
NCT02981407 {published data only}
    1. NCT02619136.Myocardial ischemia and transfusion (MINT) [Myocardial ischemia and transfusion: a pilot, multi-centre, open-label randomized controlled trial of two commonly used transfusion strategies in patients with myocardial infarction]. www.clinicaltrials.gov/ct2/show/NCT02619136 (first posted 2 December 2015). [CTG: ]
    1. NCT02981407.Myocardial Ischemia and Transfusion (MINT). clinicaltrials.gov/ct2/show/NCT02981407 (first posted 5 December 2016). [CTG: ]
NCT03229941 {published data only}
    1. NCT03229941.Transfusion trigger after operations in high cardiac risk patients (TOP) [CSP #599 - Transfusion trigger after operations in high cardiac risk patients (TOP)]. clinicaltrials.gov/ct2/show/NCT03229941 (first posted 26 July 2017). [CTG: ]
NCT03260478 {published data only}
    1. NCT03260478.HEMOglobin Transfusion Threshold in Traumatic Brain Injury OptimizatioN: The HEMOTION Trial (HEMOTION). clinicaltrials.gov/ct2/show/NCT03260478 (first posted 24 August 2017).
NCT03309579 {published data only}
    1. English SW, Fergusson D, Chassé M, Turgeon AF, Lauzier F, Griesdale D, et al, Canadian Critical Care Trials Group.Aneurysmal SubArachnoid Hemorrhage - Red Blood Cell Transfusion And Outcome (SAHaRA): a pilot randomised controlled trial protocol. BMJ Open 2016;6(12):e012623. [CTG: ] - PMC - PubMed
    1. NCT02483351.Aneurysmal subarachnoid hemorrhage: red blood cell transfusion and outcome (SAHaRA Pilot) [Aneurysmal subarachnoid hemorrhage: red blood cell transfusion and outcome - a pilot randomized controlled trial]. www.clinicaltrials.gov/ct2/show/NCT02483351 (first received 26 June 2015). [CTG: ]
    1. NCT3309579.SAHaRA: a randomized controlled trial [Aneurysmal subarachnoid hemorrhage - red blood cell transfusion and outcome (SAHaRA): a randomized controlled trial]. clinicaltrials.gov/ct2/show/NCT03309579 (first received 13 October 2017). [CTG: ]
NCT03871244 {published data only}
    1. NCT03871244.Pilot Optimizing Transfusion Thresholds in Critically-ill Children with Anaemia (P-OpTTICCA) [Pilot Optimizing Transfusion Thresholds in Critically-ill Children with Anaemia (P-OpTTICCA): a pilot trial for a large pragmatic international parallel open-label non-inferiority randomised controlled trial]. clinicaltrials.gov/ct2/show/NCT03871244 (first posted 12 March 2019). [CTG: ]
NCT04506125 {published data only}
    1. NCT04506125.Liberal versus restrictive transfusion threshold in oncologic surgery (LIBERTY1) [Liberal versus restrictive transfusion threshold in oncologic surgery: a multicenter, randomized, controlled, pilot study]. clinicaltrials.gov/ct2/show/NCT04506125 (first posted 10 August 2020). [CTG: ]
NCT04591574 {published data only}
    1. NCT04591574.ABC - a post intensive care anaemia management trial [Anaemia management with red blood cell transfusion to improve post-intensive care disability: a randomised controlled trial (the ABC post-intensive care trial)]. https://clinicaltrials.gov/ct2/show/NCT04591574 (first accepted 19 October 2020).
NCT04754022 {published data only}
    1. NCT04754022.Transfusion requirements in younger patients undergoing cardiac surgery (TRICS-IV) [An international, multi-centre, randomized controlled trial to assess transfusion thresholds in younger patients undergoing cardiac surgery]. www.clinicaltrials.gov/ct2/show/NCT04754022 (first posted 15 February 2021). [CTG: ]
NIHR 130875 {published data only}
    1. The impact of REstrictive versus LIberal Transfusion strategy on cardiac injury and death in patients undergoing surgery for Hip Fracture (RESULT-Hip). Ongoing study.2022. Contact author for more information.
NL3090 NTR3244 {unpublished data only}https://www.trialregister.nl/trial/3090
    1. NL3090 NTR3244.Blood transfusion study in patients at risk for cardiac complications after non-cardiac surgery [Perioperative Transfusion Study (PETS): does a liberal transfusion protocol improve outcome in high-risk cardiovascular patients undergoing non-cardiac surgery?]. www.trialregister.nl/trial/3090 (first posted 17 January 2012).

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