Eleftherios C. Vamvakas
Department of Pathology and Laboratory Medicine, Cedars-Sinai Medical Center, Los Angeles, California, USA
Transfusion-related immunomodulation (TRIM) enc-ompasses the documented laboratory immune alterations that follow allogeneic blood transfusion (ABT), as well as any established or purported, beneficial or deleterious, clinical effects that may be ascribed to immunosuppression resulting from ABT, including:
· enhanced survival of renal allografts;
· increased risk of recurrence of resected malignancies and of postoperative bacterial infections;
· increased short-term (up to 3-month posttransfusion) mortality from all causes; and
· activation of endogenous CMV or HIV infection in transfused compared with untransfused patients.
Any ABT-related increase in short-term posttransfusion mortality (perhaps secondary to an increased risk of multiple organ failure, MOF) would most likely be mediated by ‘proinflammatory’ rather than ‘immunomodulatory’ mechanisms; however, the term TRIM has recently been used more broadly, to encompass transfusion complications mediated via either immunomodulatory or proinflammatory pathways.
The only established clinical TRIM effect is beneficial, not deleterious. It is the enhanced survival of renal allografts after pretransplant ABT . This effect has been confirmed by animal data and clinical experience worldwide. Before the advent of cyclosporine and potent immunosuppressive drugs, this ABT effect was exploited clinically, through the deliberate exposure of patients awaiting renal transplantation to transfusion of non-leucocyte-reduced red blood cells (RBCs). The benefit from ABT is small in the current era, but a randomized controlled trial (RCT) documented that it is still evident. The existence of deleterious clinically relevant TRIM effects has not yet been established; neither do we know the mechanism(s) of TRIM or the specific blood constituent(s) that mediate(s) TRIM. TRIM may be mediated by one (or more) of the following (Figure 10.1):
· allogeneic mononuclear cells (AMCs) present in RBC units stored for less than 2 weeks;
· proinflammatory soluble mediators released from leucocyte granules or membranes and accumulating progressively in the supernatant of RBCs during storage; and/or
· soluble, class I HLA molecules circulating in allogeneic plasma.
Fig 10.1 TRIM effects, postulated mediators of TRIM and preventive strategies that could be effective if the TRIM effects were mediated by each corresponding mediator. ANH, acute normovolemic hemodilution; IBR, intraoperative blood recovery; PBR, postoperative blood recovery. Modified with permission from Vamvakas EC. Deleterious effects of transfusion-related immunomodulation: fact or fiction? Update through 2005. Am J Clin Pathol2006; 126 (Suppl 1): S71–S85.
Clinical studies of adverse TRIM effects
Some 200 observational studies and 22 RCTs reported before 2005 examined the purported adverse TRIM effects in humans. The RBC components transfused in these studies reflected the RBC components used in Europe and North America between the mid-1980s and the first few years of the 21st century. Sixteen RCTs assumed that the TRIM effect is mediated by allogeneic leucocytes and compared recipients of non-leucocyte-reduced versus leucocyte-reduced allogeneic RBCs or whole blood. Six RCTs assumed that the TRIM effect is mediated by either allogeneic leucocytes or allogeneic plasma and compared recipients of non-leucocyte-reduced allogeneic versus autologous blood. Thus, the reported RCTs differed in ways that determined the conclusions to be drawn about mechanisms of TRIM. Patients randomized to receive non-leucocyte-reduced allogeneic RBCs received units that were either buffy-coat-reduced (in Europe) or buffy-coat-rich (in the USA). Buffy-coat-reduced RBCs are units from which approximately two-thirds of leucocytes are removed, without filtration, by the method used to separate blood into components. If leucocytes do indeed mediate TRIM, buffy-coat-rich RBCs should have more of a TRIM effect than buffy-coat-reduced RBCs.
Patients randomized to receive autologous or leucocyte-reduced allogeneic RBCs received units that were either replete with or devoid of leucocyte-derived soluble mediators. During storage of a non-leucocyte-reduced RBC unit, leucocytes deteriorate over 2 weeks, progressively releasing soluble mediators. RBCs leucocyte reduced by filtration after storage are full of leucocyte-derived mediators, because such mediators (as well as apoptotic or necrotic leucocytes) are not retained by leucocyte reduction filters. RBCs leucocyte reduced by filtration before storage are free of leucocyte-derived mediators, because their leucocytes are removed before they can release mediators into the supernatant fluid. Stored autologous blood, obtained by preoperative donation, is full of mediators, because autologous leucocytes also deteriorate during storage, releasing mediators. Fresh autologous blood, obtained by acute normovolemic haemodilution (ANH), intraoperative blood recovery (IBR) or postoperative blood recovery (PBR), and transfused within hours of collection and processing, is free of leucocyte-derived soluble mediators.
Leucocyte reduction, performed either before or after storage, can prevent TRIM effects mediated by AMCs, but it cannot prevent TRIM effects mediated by soluble molecules circulating in allogeneic plasma (Figure 10.1). Only pre-storage, as opposed to post-storage, leucocyte reduction can prevent TRIM effects mediated by leucocyte-derived, soluble mediators. Autologous transfusion can prevent TRIM effects mediated by AMCs as well as by molecules circulating in allogeneic plasma. Only fresh, as opposed to stored, autologous blood can prevent TRIM effects mediated by leucocyte-derived, soluble mediators.
TRIM effects mediated by AMCs
The best argument that the adverse TRIM effects are mediated by AMCs is that the established beneficial TRIM effect in renal transplantation requires viable leucocytes. Also, immune suppression has been induced in mice receiving allogeneic leucocytes free of plasma and platelets. Viable allogeneic leucocytes in blood components can act as responder cells or as stimulator cells, inducing cellular immunity and antibody production in the recipient. After 10–14 days of storage, the capacity of donor antigen-presenting leucocytes to stimulate recipient T-helper cells is abrogated in vitro owing to a reduction in costimulatory molecules.
The only RCT to study the effect of AMCs has been the Viral Activation Transfusion Study, which studied transfusion-induced activation of endogenous CMV or HIV infection . All RBCs transfused in this study had been stored for less than 2 weeks and could thus be presumed to contain immunologically competent AMCs. There was no difference between the arms of the RCT in the HIV–RNA level, the number of CD4-positive T cells or any other endpoint studied. Median survival was 13.0 months in recipients of pre-storage filtered, leucocyte-reduced allogeneic RBCs, as compared with 20.5 months in recipients of buffy-coat-rich, non-leucocyte-reduced allogeneic RBCs (p= 0.12). This difference was not significant in the intention-to-treat analysis but, after correction for various prognostic factors, transfusion of non-leucocyte-reduced RBCs was associated with a better outcome.
TRIM effects mediated by leucocyte-derived soluble mediators
Soluble immune response modifiers accumulating during storage of blood components include elastase, histamine, soluble HLA, soluble Fas ligand, transforming growth factor (TGF)-β1 and proinflammatory cytokines IL-1β, IL-6 and IL-8. In vitro, soluble leucocyte-derived factors from stored RBCs induce immediate upregulation of expression of inflammatory genes in third-party leucocytes . Apoptosis of leucocytes begins immediately after the collection of donor blood. Gradual apoptosis and necrosis begins with granulocytes and continues with monocytes, while lymphocytes can remain viable for >25 days at 2–6°C. Apoptotic cells engage the phosphatidylserine/annexin-V receptor on macrophages, inducing release of prostaglandin E-2 and TGF-β – factors that suppress macrophages and natural killer cells and impair antigen-presenting capacity.
However, the 12 RCTs that compared the risk of postoperative infection between patients randomized to receive non-leucocyte-reduced versus leucocyte-reduced ABT (in the event that they needed perioperative transfusion) have not supported the theory that attributes TRIM to leucocyte-derived soluble mediators. These RCTs are medically heterogeneous, having been conducted at various settings, having transfused various blood components and having diagnosed infection based on varying criteria. Thus, not all 12 RCTs targeted a TRIM effect that was biologically similar in all cases, making it inappropriate to combine the results of all 12 RCTs in a meta-analysis. However, if we were to integrate these findings despite the extreme heterogeneity of the studies, we would find no association between non-leucocyte-reduced ABT and an increased risk of infection across all the available RCTs, whether we relied on ‘intention-to-treat’ analyses (that retain all randomized subjects, whether transfused or not) or on ‘as-treated’ analyses (that remove the untransfused subjects) .
What has medical relevance, however, is the integration of medically homogeneous studies. Integration of such subsets of homogeneous studies produced results antithetical from those expected from theory. Across nine RCTs transfusing allogeneic RBCs filtered before storage to the leucocyte-reduced arm and enrolling approximately 5000 subjects, no TRIM effect was detected. If leucocyte-derived, soluble mediators did cause TRIM, pre-storage filtration should abrogate any increased infection risk associated with non-leucocyte-reduced ABT. Thus, a deleterious TRIM effect would be expected in this analysis, but no such effect was found (summary odds ratio (OR) = 1.06; 95% confidence interval (CI), 0.91–1.24; p > 0.05) (Figure 10.2a).
In contrast, across the four RCTs transfusing allogeneic RBCs or whole blood filtered after storage to the non-leucocyte-reduced arm, there was a 2.25-fold increase in the risk of infection in association with non-leucocyte-reduced ABT (summary OR = 2.25; 95% CI, 1.12–4.25; p < 0.05) (Figure 10.2b). Thus, the TRIM effect appeared to be prevented by post-storage filtration, contradicting the theory that attributes TRIM to leucocyte-derived, soluble mediators. Such mediators would have been present equally in both the non-leucocyte-reduced and leucocyte-reduced RBCs, because they would not have been removed by post-storage filtration.
TRIM effects mediated by soluble molecules circulating in allogeneic plasma
Only one RCT has been specifically designed to study the effects of soluble HLA molecules circulating in allogeneic plasma as mediators of TRIM. Wallis et al.  randomized patients undergoing open-heart surgery to receive plasma-reduced, buffy-coat-reduced or leucocyte-reduced RBCs. The highest risk of infection was observed in the plasma-reduced arm, although the difference between the three arms was not significant. This finding suggested that plasma removal does not prevent TRIM or, by extension, that allogeneic plasma does not mediate TRIM. Similarly, integration of the five RCTs that compared recipients of allogeneic versus autologous blood demonstrated no increased risk of infection in association with ABT.
Association between non-leucocyte-reduced ABT and short-term mortality
The association of non-leucocyte-reduced ABT with short-term mortality from all causes started out as a data-derived hypothesis to account for an unexpected transfusion effect. The RCT of van de Watering et al.  (Figure 10.2) had been designed to investigate differences in postoperative infection between recipients of non-leucocyte-reduced versus leucocyte-reduced allogeneic RBCs. However, it detected, instead, differences in 60-day mortality between the arms (Figure 10.3). The authors suggested that non-leucocyte-reduced ABT may predispose to MOF, which, in turn, may predispose to mortality.
Fig 10.2 RCTs of ABT and postoperative infection administering (a) pre-storage leucocyte-filtered or (b) post-storage leucocyte-filtered allogeneic RBCs to the leucocyte-reduced arm. The figure shows the OR of postoperative infection, as calculated from an intention-to-treat analysis of each study, and the summary OR across the depicted RCTs, as calculated from a meta-analysis. A deleterious ABT effect (and thus a benefit from leucocyte reduction) is demonstrated by an OR > 1, provided that the effect is statistically significant (p < 0.05; i.e. provided that the associated 95% CI does not include the null value of 1). The RCT of van de Watering et al. included recipients of both pre-storage filtered and post-storage filtered RBCs and found no difference between these two arms. For the references to the listed studies, see Further reading, Vamvakas & Blajchman (2007).
Fig 10.3 RCTs investigating the association of non-leucocyte-reduced ABT with (a) short-term (up to 3-month posttransfusion), all-cause mortality and transfusing buffy-coat-reduced versus pre-storage filtered allogeneic RBCs or (b) conducted in cardiac surgery. For each RCT, the figure shows the OR of short-term mortality, as calculated from an intention-to-treat analysis and the summary OR across the depicted RCTs, as calculated from a meta-analysis. A deleterious ABT effect (and thus a benefit from leucocyte reduction) is demonstrated by an OR > 1, provided that the effect is statistically significant (p < 0.05; i.e. provided that the associated 95% CI does not include the null value of 1). For the references to the listed studies, see Further reading, Vamvakas & Blajchman (2007).
If 11 medically heterogeneous RCTs reported before 2005 and comparing recipients on non-leucocyte-reduced versus leucocyte-reduced ABT and reporting on short-term mortality were to be combined, there would be no increase in mortality in association with non-leucocyte-reduced ABT. These studies transfused to the non-leucocyte-reduced arm either buffy-coat-rich or buffy-coat-reduced allogeneic RBCs; as already discussed, the former should have more of an effect than the latter. However, this theoretical prediction is the opposite of what the analysis actually showed. Across six RCTs transfusing buffy-coat-reduced RBCs to the non-leucocyte-reduced arm and pre-storage filtered RBCs to the leucocyte-reduced arm, there was a 60% increase in mortality in association with non-leucocyte-reduced ABT (summary OR = 1.60; 95% CI, 1.14–2.24; p < 0.05) (Figure 10.3a). In this analysis, pre-storage filtration appeared to abrogate an increased mortality risk, but the benefit from pre-storage filtration was not seen where more of an ABT effect would have been expected. Across the RCTs that transfused buffy-coat-rich RBCs to the non-leucocyte-reduced arm, no ABT effect was detected, although some 4500 subjects had been enrolled in these studies.
Perhaps the benefit observed in the analysis of studies transfusing buffy-coat-reduced versus pre-storage filtered RBCs (Figure 10.3a) was due to overrepresentation in that analysis of the cardiac surgery studies: three of the six RCTs included in that analysis (i.e. the studies by van de Watering et al. , Bilgin et al.  and Wallis et al. ) had been conducted in open-heart surgery. Across all five RCTs conducted in cardiac surgery (Figure 10.3b), there was a 72% increase in mortality in association with non-leucocyte-reduced ABT (summary OR = 1.72; 95% CI, 1.05–2.81; p < 0.05). In contrast, across the six RCTs conducted in other surgical settings, there was no ABT effect (summary OR = 0.99; 95% CI, 0.73–1.33; p > 0.05).
Thus, the ABT-related mortality risk, which is not seen in any other setting, may relate to another effect present in patients undergoing cardiac surgery. During open-heart surgery, blood is exposed to the extracorporeal circuit, as well as to hypothermia and to ischemic and reperfusion injury. These insults are potent inducers of a stress response, triggering a systemic inflammatory response syndrome (SIRS), which is immediately counteracted by a compensatory anti-inflammatory response syndrome (CARS) . SIRS manifests with leukocytosis, capillary leakage and organ dysfunction; overwhelming SIRS causes a dormant state of metabolism referred to as multiple-organ dysfunction syndrome (MODS). CARS has an immune-paralysing effect characterized by anti-inflammatory cytokines, such as TGF-β1, IL-4 and IL-10. Through the intervention of CARS, the postperfusion SIRS of cardiac surgery generally resolves. However, any intervention by biologic response modifiers during an already existing inflammatory cascade can push the SIRS/CARS equilibrium towards SIRS, thereby leading to MODS, MOF and death. Leucocyte-containing ABT administered during cardiac surgery may provide this ‘second hit’, exacerbating the SIRS and potentially causing the patient's death . However, this explanation is speculative as, hitherto, no cardiac surgery RCT has reported an association between non-leucocyte-reduced (versus leucocyte-reduced) ABT and MOF.
Adverse effects of the transfusion of ‘old’ RBCs
Since 2005, the controversy about the deleterious effects of ‘old’ (versus ‘fresh’) RBCs has supplanted the debate on the purported adverse TRIM effects. When non-leucocyte-reduced ‘old’ RBCs are considered, some of the same postulated biologic mechanisms (e.g. the effects mediated by leucocyte-derived, soluble mediators) are invoked for both the adverse TRIM effects and the adverse effects of old RBCs. For both the non-leucocyte-reduced and leucocyte-reduced ‘old’ RBCs, additional mechanisms have been proposed for the association between transfusion of old RBCs and increased risk of short-term mortality, in-hospital infection or MOF, which include the following:
· proinflammatory, procoagulant and/or immune effects of old (rather than fresh) RBCs secondary to the development of microparticles in old blood;
· an increase in iron load from haemolysed stored RBCs;
· activation of complement and neutrophils and/or depletion of nitric oxide or S-nitrosylated hemoglobin in stored RBCs, which reduces the ability of the transfused RBCs to induce vasodilation (thereby resulting in inadequate blood flow and impaired oxygen delivery).
In the words of Ness , the question whether ‘old’ RBCs are less safe than fresh RBCs is the most critical issue facing transfusion medicine today. If the associations of old (rather than fresh) RBCs with increased mortality, infection and/or MOF reported from observational studies are shown to be causal, the allowed storage period of RBCs (35 or 42 days) will have to be promptly reduced – depending on the findings of future studies – to 2, 3 or 4 weeks. In addition, greater reliance will have to be placed on approaches to patient blood management , including the appropriate conservation and management of a patient's own blood as a vital resource, because the patient's own freshly shed blood is the freshest blood possible. Several large RCTs comparing the frequency of common adverse outcomes between recipients of ‘old’ and ‘fresh’ RBCs are currently enrolling patients in North America.
Some experts have argued that it is injudicious to wait for the results of these RCTs before we reduce the allowed RBC storage period. Instead of waiting for the definitive evidence that the ongoing RCTs will provide, inference of cause should be made from passive observation (i.e. from the results of the available observational studies), so that policy decisions can be made in the manner that they are made in risk-factor epidemiology – an area in which policymakers deal with deleterious exposures such as asbestos or tobacco smoke . A widely publicized, large retrospective study published in a prominent journal reported an association between transfusion of RBCs older than 14 days and significantly increased mortality . However, the conclusions reached by the numerous available observational studies are not unanimous and the hitherto-completed pilot RCTs have produced no evidence that transfusion of ‘old’ (rather than ‘fresh’) RBCs is associated with increased mortality.
Most observational studies reported on the effect of a single RBC unit: the oldest RBC unit transfused during a patient's hospitalization. Yet, observational studies suffer from an inability to separate any effect of the oldest transfused RBC unit from the effect of the number of transfused RBCs. Because need for RBC transfusion reflects the presence of comorbidities, more severe illness and a poorer baseline prognosis, the number of transfused RBCs is the best predictor of adverse outcomes in observational studies. In addition, the number of transfused RBCs is associated – nonspecifically – with any adverse outcome (be it mortality or major morbidity such as in-hospital infection or MOF), because it reflects – nonspecifically – more severe illness. As illustrated by van de Watering et al. , as patients receive a total transfusion dose of 1, 2, 3, …, 10, or >10 RBC units, the oldest transfused RBC unit gets, progressively and without fail, older and older. This is because each time a request for transfusion is received by the blood bank, the blood bank issues for transfusion the oldest compatible unit in the inventory. Therefore, the more often a patient is transfused, the more likely he/she is to be issued blood at a time that the oldest compatible unit is ‘old’.
To establish a valid association between the length of storage of the oldest transfused unit and adverse outcomes in observational studies, it is therefore necessary to adjust any reported relationship for the number of transfused RBCs. A meta-analysis showed that integration of adjusted results from the observational studies published before 31 December 2008 (which had reported on the same outcome – mortality or infection or MOF – in the same clinical setting and had defined transfusion of ‘old’ versus ‘fresh’ RBCs in the same manner) produced summary results across the studies that were negative (showing no effect of old blood) in six of eight analyses . The two situations in which old blood was associated with a worse outcome did not integrate results of studies that had adjusted for the effect of the most important confounder (i.e. the number of transfused RBCs). Three additional systematic reviews concluded that the existence of the purported common adverse effects of ‘old’ RBCs cannot be adequately inferred from the available data [15–17].
Edgren et al. included all patients receiving RBCs in Denmark and Sweden between 1995 and 2002, thereby including 404 959 transfusion episodes in their study . These investigators found no effect of ‘old’ RBCs on short-term (7-day) mortality – an outcome comparable to the in-hospital mortality recorded by the other observational studies. With >400 000 transfusions in their database, Edgren et al. were able to use elaborate statistical modelling to adjust for the effects of confounding factors (including the number of transfused RBCs, as they reported separately on recipients of 1–2, 3–4 or ≥5 units, as well as on all patients and on recipients of leucocyte-reduced units); and they offered unprecedented statistical precision, as well as maximum generalizability of their findings.
Three pilot RCTs, intended to investigate the feasibility of conducting large RCTs of the relationship between transfusion of ‘old’ (rather than ‘fresh’) RBCs and common adverse outcomes, have reported data on the mortality of recipients of ‘old’ versus ‘fresh’ RBCs. All three studies showed no difference in mortality between the two arms and no trial showed a trend towards increased mortality in association with the receipt of ‘old’ (rather than ‘fresh’) RBCs.
TRIM appears to be a real biologic phenomenon resulting in at least one established beneficial clinical effect in humans, the enhanced survival of renal allografts in patients receiving pretransplant ABT, but the existence of deleterious clinical TRIM effects manifest across other clinical settings has not yet been confirmed by adequately powered RCTs. Except for cardiac surgery, there is no setting where the results of the RCTs of deleterious TRIM effects have been consistent. In cardiac surgery patients, the use of non-leucocyte-reduced ABT has been consistently associated with increased mortality, but, even in this setting, the reasons for the excess deaths remain elusive .
The other TRIM effects shown in Figures 10.2b and 10.3a, the one on postoperative infection prevented by post-storage filtration and the one on short-term mortality mediated by non-leucocyte-reduced ABT of buffy-coat-reduced RBCs, appear to contradict current theories about TRIM pathogenesis, because they are not accompanied by similar (or larger) ABT effects prevented by pre-storage filtration or mediated by buffy-coat-rich RBCs . The effect prevented by post-storage filtration may be due to the inclusion in that analysis of two early studies by Jensen et al. (Figure 10.2b) of transfused blood components no longer used today (non-leucocyte-reduced versus leucocyte-reduced allogeneic whole blood or post-storage filtered, leucocyte-reduced allogeneic RBCs). These studies reported extraordinarily large adverse TRIM effects (Figure 10.2b). No TRIM effect is detected if these studies are excluded from the meta-analysis.
The effect on mortality mediated by buffy-coat-reduced RBCs may be due to overrepresentation in that analysis of the cardiac surgery RCTs. Thus, the only adverse TRIM effect of non-leucocyte-reduced ABT that has been clinically documented in humans is increased mortality in cardiac surgery. Until further studies are conducted to pinpoint the mechanisms for these excess deaths (or to refute this association), all cellular blood components transfused in cardiac surgery should be leucocyte-reduced components. At this time, the totality of the evidence from RCTs does not support a policy of universal leucocyte reduction introduced specifically to prevent TRIM, although universal leucocyte reduction can be justified based on other clinical benefits of leucocyte reduction.
Dzik et al.  tested the benefit accrued from providing leucocyte-reduced (versus non-leucocyte-reduced) RBCs to all 2780 patients who were transfused over 6 months at a tertiary-care medical centre. No difference in mortality, use of antibiotics or hospital stay was found when leucocyte-reduced (versus non-leucocyte-reduced) RBCs were given indiscriminately to all patients, regardless of disease classification or indication for transfusion. The study was criticized because of a high (20%) proportion of protocol violations, whereby the wrong (as opposed to the assigned) RBC component was given to the enrolled patients; such protocol violations would have biased the estimate of the effect towards the null.
The evidence for implementing universal leucocyte reduction for the prevention of the TRIM effects may not be available, because the requisite adequately powered studies have not been conducted. The design of the available RCTs has not been based on specific hypotheses about the mechanisms of TRIM formulated in the preclinical studies. For example, animal models of ABT and tumour growth have convincingly documented that allogeneic blood containing functional dendritic cells can facilitate the growth of selected tumours. However, none of the available RCTs has transfused fresh, non-leucocite-reduced RBCs to test this theory or has enrolled patients with tumours whose growth could be augmented by ABT. Furthermore, to separate the TRIM mechanism in surgical patients from the TRIM mechanism in renal transplantation, Dzik et al.  proposed the existence of two different categories of TRIM effects: one operating on the innate immune response and one operating on the adaptive antigen-driven immune system. Although the innate and adaptive immune systems are linked by subsets of natural killer and dendritic cells, the difference in the clinical condition of a surgical patient versus a patient in a steady-state disease may be important and account for the various TRIM effects reported from different settings.
The latest shift in research focus has been from the investigation of the adverse TRIM effects to the investigation of the adverse effects of ‘old’ RBCs. Despite impressions that have been generated in North America from highly publicized results of observational studies, there is a paucity of evidence on any association between transfusion of ‘old’ (versus ‘fresh’) RBCs and the common adverse outcomes of mortality, in-hospital infection or MOF. Given this paucity of evidence, it would be injudicious for policy-makers to endorse any radical change in policy (mandating transfusion of ‘fresh’ rather than ‘old’ RBCs) without waiting for the definitive evidence to be produced by the RCTs currently underway in North America.
1. TRIM is a real biologic phenomenon, resulting in at least one established beneficial clinical effect in humans (enhancement of renal allograft survival).
2. The existence of deleterious clinical TRIM effects has not yet been confirmed by adequately powered RCTs.
3. In cardiac surgery, transfusion of leucocyte-reduced (compared with non-leucocyte-reduced) allogeneic RBCs appears to reduce short-term (up to 3 months posttransfusion) all-cause mortality, but the reasons for the observed excess deaths remain elusive.
4. Based on the available data, leucocyte reduction of all cellular blood components – introduced specifically for the prevention of adverse TRIM effects – is indicated in cardiac surgery.
5. There is a paucity of evidence on any association between transfusion of ‘old’ RBCs and the common adverse outcomes of mortality, in-hospital infection or MOF.
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