Red blood cell transfusion, lowest haemoglobin level, and their association with mortality and length of hospital stay in surgical patients.

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Anaemia, a global public health problem, is common in developing and developed countries, particularly among hospitalized patients. The World Health Organization defines anaemia as a haemoglobin concentration below 130 grams per litre in males and 120 grams in litre in females. A recent systematic review and meta-analysis on preoperative anaemia and outcomes after cardiac and non-cardiac surgery reported 39% of patients were admitted anaemic. These anaemic patients had three-fold higher odds of mortality, four-fold higher odds of acute kidney injury and twice the odds of infection. Not surprisingly, anaemia was also associated with increased transfusion, with anaemic patients five-times more likely to receive a red blood cell transfusion. As these results indicate, red blood cell transfusions are often administered to correct low haemoglobin levels. However, correcting anaemia with transfusion is problematic as red blood cell transfusion has a dose-dependent association with increased mortality, morbidity, hospital and ICU length of stay, readmissions, and cost. Large risk-adjusted observational studies demonstrate that even transfusing a single unit of red blood cells is associated with increased adverse outcomes in surgical patients, thus recommending caution before transfusing. In an attempt to find the “optimal” transfusion threshold, many randomized controlled trials have investigated the difference between using restrictive pre-transfusion haemoglobin thresholds compared with liberal thresholds. A restrictive strategy refers to a policy of administering red blood cell transfusions at comparatively lower pre-defined haemoglobin levels, with the goal of minimizing the use of blood. Though not always the case, restrictive transfusion thresholds are often defined as haemoglobin levels between 70 grams per litre and 80 grams per litre and liberal transfusion thresholds are commonly defined as haemoglobin levels between 90 grams per litre and 100 grams per litre. A recent systematic review and meta-analysis published in the Cochrane Library concluded there is no difference in morbidity or mortality between restrictive and liberal transfusion strategies. However, these trials are often confounded by transfusions administered pre-randomization, a lack of comparable transfusion dosing regimens between studies, and at times small differences in actual mean pre-transfusion haemoglobin levels between control and intervention arms. In addition, these randomized controlled trials do not address transfusion efficacy and still leave many important questions unanswered. For example, whether haemoglobin thresholds lower than 70 grams per litre are just as effective as haemoglobin levels higher than 70 grams per litre. Some have suggested lower haemoglobin thresholds may be just as effective. The aim of this study is to determine what effect red blood cell transfusion has on mortality and length of stay at various levels of nadir haemoglobin.

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