Introduction
Anemia is extremely common in the critically ill [
1] and is associated with poor outcomes [
2‐
5]. It is therefore not surprising that 19 to 53% of all patients admitted to adult ICUs receive at least one unit of allogeneic red blood cells (RBCs) [
1,
6‐
8].
Several publications have highlighted that the administration of RBCs and the hemoglobin trigger used for the administration of RBCs may affect patient morbidity and mortality [
9‐
18]. More recently, the age of RBCs has been the focus of concern as a potential cause of increased morbidity and mortality [
10]. A recent review summarizing data from 27 different studies in adult patients, however, concluded that it is difficult to determine whether there is a relationship between the age of transfused RBCs and mortality [
19].
The mechanism responsible for the possible adverse effects of RBCs may relate to the development of storage lesions over time. During storage, in a way that increases over time, important biochemical changes occur: a reduction in 2,3-diphosphoglycerate, hypocalcemia, cell lysis, release of free hemoglobin, changes in nitric oxide levels, alterations in pH [
20,
21], and increases in lipids [
22], complement [
23] and cytokines [
24]. These changes are accompanied by increased membrane fragility, which can compromise microcirculatory flow and lead to increased red cell-endothelial cell interaction and inflammatory cytokine release [
20,
21]. Such changes, which serve as potential explanations for more unfavorable outcomes, may be particularly disadvantageous to critically ill patients with a higher mortality risk. In this group, indirect evidence has linked the transfusion of older RBCs with adverse clinical consequences [
25]. Unfortunately, all such evidence has been retrospective and/or focused on specific patient groups. The robustness of the relationship between the age of RBCs and adverse clinical outcome is thus limited both in strength and generalizability. Yet if this link exists, the public health consequences are great, given that the transfusion of RBCs is a common treatment in the critically ill. Furthermore, exposure to even a single unit of older RBCs might be associated with unfavorable outcome independent of the effect of volume of transfused RBCs and other confounding factors.
Accordingly, we hypothesized that the maximum age of RBCs to which a critically ill patient had been exposed would have an independent relationship with hospital mortality. We tested this hypothesis by conducting a prospective multicenter observational study in a heterogeneous group of medical and surgical critically ill patients.
Discussion
We conducted a prospective observational study in 47 ICUs in Australia and New Zealand to assess the association between age of RBCs and outcome. In critically ill patients receiving RBCs, we found an association between exposure to older red cells and increased hospital mortality rate. This association remained after adjustment for potential confounding factors.
In this study, the mean age of all RBCs was 16.2 days and the oldest RBC unit given to each patient was 19.6 days on average. This compares with 21.2 days in the United States [
1] and 16.2 days in Europe [
7]. In 2007, the mean calculated age of transfused RBCs in the United States was 19.5 days, although just 7.8% of the hospitals reported such data [
27]. Our results, therefore, are in agreement with the mean age of RBCs in previous studies and in other countries.
The mean pretransfusion hemoglobin values in previous studies - namely 8.6 g/dl in the United States [
1] and 8.4 g/dl in Europe [
7] - are in line with our mean pretransfusion hemoglobin concentration. In a previous study in Australia and New Zealand conducted in 2001 by French and colleagues the median pretransfusion hemoglobin level was 8.2 g/dl [
6], compared with 7.7 g/dl in the present study. In keeping with published evidence [
9], therefore, Australian and New Zealand transfusion practice appears to have moved toward a more restrictive approach during recent years.
There is no suitably powered randomized controlled trial of the effect of age of RBCs on mortality [
28]. Moreover, with the exception of cardiac surgery patients, no prospective cohort study of adequate sample size has evaluated the possible association between RBC age and mortality in the critical care setting. In trauma patients, four small single-center cohort studies have suggested that exposure to older RBCs may be an independent risk factor for multiple organ dysfunction [
29], increased infections [
14], and increased ICU length of stay [
30] and hospital length of stay [
31], but none have assessed its link with mortality. Our prospective multicenter cohort study is therefore the first to assess the independent relationship between the age of RBCs and hospital mortality in a heterogeneous population of critically ill patients. Nonetheless, our findings must be seen in light of three recent large retrospective studies in cardiac surgery patients [
10], in trauma patients [
32], and in a registry of hospitalized patients [
33].
In a study of 6,002 cardiac surgery patients, Koch and colleagues found that patients given older RBCs had an increase in unadjusted mortality, prolonged ventilation and increased sepsis, and that the transfusion of older RBCs was independently associated with an increased risk-adjusted rate of a composite of serious adverse events [
10]. Although the findings of the above study are both important and provocative and the sample size was large, several features of its design made confirmatory studies desirable. First, the study was retrospective with all the inherent shortcomings of such a design. Second, the study focused only on cardiac surgery patients. Third, the study excluded more than 28% of patients because those patients received both fresh and older RBCs. Fourth, the study separated patients into two groups only according to the age of RBCs using an arbitrary 14-day cut-off point. Finally, the study did not adjust for baseline differences, age or number of units transfused before ICU treatment, and combined intraoperative and postoperative RBC transfusions [
26,
34].
Recently, Weinberg and colleagues demonstrated a higher mortality among trauma patients who received at least three RBC units [
32]. In concordance, the largest registry study in recipients of RBC transfusion from 1995 to 2002 by Edgren and colleagues suggested that RBCs older than 30 days were associated with an increased risk of death in a 2-year follow-up [
33].
Whilst impressive in sample size the retrospective registry studies have been performed mostly outside the critical care setting with a lower expected mortality rate and, thus, a lesser ability to detect relative reduction in risk. Therefore, because of the limitations of the previous studies and the public health importance of this issue, we considered it desirable to conduct a prospective, multicenter study to confirm or refute these findings in a broader population of critically ill patients.
We initially found a difference in unadjusted mortality rates according to the maximum age of red cells to which a patient had been exposed: the quartiles with older red cells were associated with a clear increase in mortality when compared with the lowest RBC quartile. However, we reasoned that this difference required correction for illness severity. Accordingly, to more rigorously test the validity of our findings, we performed multivariate analysis in these patients. We adjusted for both APACHE III score, number of transfusions, pre-ICU transfusions, fresh frozen plasma and platelet transfusions, leukodepletion status, pretransfusion hemoglobin concentration, clustering of study sites, and cardiac surgery, and we used hospital mortality as the dependent variable and found a significant and independent association between the maximum age of red cells to which a patient had been exposed and mortality. Our findings indicating an association between exposure to older RBCs and increased mortality are in broad agreement with the results of the three large retrospective studies [
10,
32,
33], and with a
post hoc analysis of a randomized controlled trial in critically ill children by Gauvin and colleagues [
35]. The association between higher transfusion hemoglobin and higher mortality may reflect physician attempts to compensate for more severe underlying disease (for example, chronic pulmonary or cardiovascular or cerebrovascular disease) or ongoing bleeding.
The present study has several strengths. The investigation was a prospective, multicenter study and included a heterogeneous group of critically ill patients, increasing its generalizability. In addition, the study included multivariate adjustment for baseline characteristics, illness severity and relevant variables using in-hospital mortality as an endpoint.
The study also, however, has some significant limitations. This study was not a randomized trial, thus any association detected by multivariate regression analysis does not imply causation. For example, there may have been factors that influenced this association of which we are not aware and were unable to correct for (for example, use of vasopressors, PaO
2/FiO
2 ratios, use of antibiotics). Treating clinicians were not blinded to the age of RBCs. We have no reason to believe, however, that clinician behavior was influenced by or itself influenced the age of transfused RBCs, a variable outside their control. We did not obtain data on red cell transfusion outside the ICU. We did not follow-up patients after hospital discharge to establish their 90-day survival; such follow-up might have affected our findings. The study comprised only Australian and New Zealand ICUs and its findings may not apply to other healthcare systems. The transfusion practice and the mean age of transfused red cells, however, appear similar to those reported in studies from Europe and North America. The maximum age of red cells was not significantly associated with hospital mortality when evaluated as a continuous variable, but had a significant association when evaluated using quartiles, which can be explained by the nonlinear association demonstrated in Figure
1. In addition, our exploratory
post hoc analysis suggests that a linear relationship between the age of blood and mortality may exist for RCBs with a lower maximum age (<15 days old), but that, beyond approximately 15 days, the deleterious effects may be less. The missing linear relationship across the whole range of RBC's age is biologically plausible given the possibility of a maximum level of deleterious changes in RBCs over time. It is also conceivable that the use of a maximum value may not readily lend itself to a linear relationship. Finally, the unadjusted difference in hospital mortality was high, raising some uncertainty about biological plausibility. In response, we adjusted for all relevant available confounding factors, expecting the difference to lose statistical significance; it did not.
Acknowledgements
The authors would like to thank the Australian Red Cross Blood Service and the New Zealand Blood Service for excellent collaboration during this study, and the Australian and New Zealand Intensive Care Society Centre for Outcome and Resources Evaluation Adult Patient Database for the APACHE III data.
Unrestricted grants were received from the Australian Red Cross Blood Service, and in-kind support from the Australian and New Zealand Intensive Care Research Centre.
The present study is a collaboration of the Australian and New Zealand Intensive Care Society Clinical Trials Group, the Australian Red Cross Blood Service, and the New Zealand Blood Service. The Blood Observational Study Writing Committee takes responsibility for the content and integrity of the present article.
Blood Observational Study Writing Committee: V. Pettilä (Chair), A. Westbrook (Chair), A. Nichol, M.J. Bailey, E. Wood, G. Syres, L.E. Phillips, A. Street, C. French, L. Murray, N. Orford, J. Santamaria, R. Bellomo, and D.J. Cooper.
The Blood Observational Study site investigators are as follows (alphabetical order - all in Australia unless specified): Alfred Hospital, Melbourne - D.J. Cooper, A. Nichol, A. Street, S. Vallance; Auckland City Hospital, Auckland, New Zealand - C. McArthur, S. McGuiness, L. Newby, C. Simmonds, R. Parke, H. Buhr; Austin Health, Melbourne - R. Bellomo, D. Goldsmith, K. O'Sullivan, I. Mercer; Ballarat Health Services, Ballarat - R. Gazzard, C. Tauschke, D. Hill; Bendigo Hospital, Bendigo - J. Fletcher, C. Boschert, G. Koch; Box Hill Hospital, Melbourne - D. Ernest, S. Eliott, B. Howe; Cabrini Private Hospital, Melbourne - F. Hawker; Calvary Mater Newcastle Hospital, Waratah - K. Ellem, K. Duff; Christchurch Hospital, Christchurch, New Zealand - S. Henderson, J. Mehrtens; Concord Hospital, Concord - D. Milliss, H. Wong; Dandenong Hospital, Dandenong - S. Arora, B O'Bree, K. Shepherd; Epworth Eastern, Melbourne - B. Ihle, S. Ho; Epworth Richmond, Melbourne - B. Ihle, M. Graan; Flinders Hospital, Bedford - A. Bernsten, E. Ryan. Frankston - J. Botha, J. Vuat; The Geelong Hospital, Geelong - N. Orford, A. Kinmonth, M. Fraser; Gold Coast Hospital, Southport - B. Richards, M. Tallott, R. Whitbread; Hawke's Bay Hospital, Hastings, New Zealand - R. Freebairn, A. Anderson; Liverpool Hospital, Liverpool - M. Parr, S. Micallef; Lyell McEwin, Elisabeth Vale - K. Deshpande, J. Wood; Middlemore Hospital, Auckland, New Zealand - T. Williams, J. Tai, A. Boase; Monash Medical Centre, Melbourne - S. Arora, P. Galt; Nelson Hospital, Nelson, New Zealand - B. King, R. Price, J. Tomlinson; Nepean Hospital, Penrith - L. Cole, I. Seppelt, L. Weisbrodt, R. Gresham, M. Nikas; North Shore Hospital, Auckland, New Zealand - J. Laing, J. Bell; Palmerston North Hospital, Palmerston, New Zealand - G. McHugh, D. Hancock, S. Kirkman; Prince of Wales Hospital, Randwick - Y. Shehabi, M. Campbell, V. Stockdale; Queen Elisabeth Hospital, Adelaide - S. Peake, P. Williams; Royal Adelaide Hospital, Adelaide - P. Sharley, S. O'Connor; Royal Darwin Hospital, Darwin - D. Stephens, J. Thomas; Royal Hobart Hospital, Hobart - R. Sistla, R. McAllister, K. Marsden; Royal Melbourne Hospital, Melbourne - C. MacIsaac, D. Barge, T. Caf; Royal North Shore Hospital, Sydney - S. Finfer, L. Tan, S. Bird; Royal Perth Hospital, Perth - S. Webb, J. Chamberlain, G. McEntaggart, A. Gould; Royal Prince Alfred Hospital, Sydney - R. Totaro, D. Rajbhandari; Sir Charles Gairdner Hospital, Nedlands - S. Baker, B. Roberts; St Andrew's War Memorial Hospital, Brisbane - P. Lavercombe, R. Walker; St George Hospital, Sydney - J. Myburgh, V. Dhiacou; St Vincent's Hospital, Melbourne - J. Santamaria, R Smith, J. Holmes; St Vincent's, Sydney - P. Nair, C. Burns; Tauranga Hospital, Tauranga, New Zealand - T. Browne, J. Goodson; Waikato Hospital, Hamilton, New Zealand - F. van Haren, M. La Pine; Warringal Private, Heidelberg - G. Hart, J. Broadbent; Wellington Hospital, Wellington, New Zealand - P. Hicks, D. Mackle, L. Andrews; Western Hospital, Melbourne - C. French. H. Raunow, L. Keen; and Wollongong Hospital, Wollongong - A. Davey-Quinn, F. Hill, R. Xu.
Authors' contributions
AJW, ADN, MJB, DJC, GS, EMW, AS, CF and RB were involved in the study design. GS, LM, AJW, ADN, JDS, NO and VP collected the data. MJB performed the statistical analysis. VP and RB drafted the first manuscript. All authors participated in drafting and revision of the manuscript. All authors were involved in data acquisition, and read and approved the final manuscript.