Blood: a very special juice. The good and the evil.

In Goethe's Faust, Mephistopheles stated, 'Blood is a juice of very special kind.' As illustrated in this famous German folklore, blood is the essence of life but it can also lead to the destruction of life if improperly used. The human heart constantly pumps on average between 5 and 6 l of blood throughout the body to maintain the supply of nutrients and oxygen to organs and cells, which is essential for life. Indeed, Mephistopheles was correct in his observation that when we start to talk about blood, we are talking about something very special. The question to initiate red blood cells (RBCs) transfusion is a daily clinical decision in the operating theatre, as the most secure strategy for managing patients with low haemoglobin has not yet been properly established. Moreover, the definition of low haemo-globin concentration thresholds remains controversial [1]. It is with certainty that markers are needed to help clinicians identify whether a patient would benefit from blood transfusion. This remains a relevant research topic and is in line with the fact that we definitely have a lot to learn about erythrocyte transfusion. Willems et al. [2] investigated this important issue in a large retrospective study, looking at the outcome of children after cardiac surgery. They hypothesized that the timing of blood transfusion plays a key role in the difference in patient outcomes. The conclusion of the study showed that transfusions administered therapeutically were associated with increased morbidity and mortality, compared with transfusions administered simply to avoid a low haematocrit during cardiopulmonary bypass (CPB). On the other hand, higher preoperative weight was associated with a lower risk of severe postoperative morbidity or mortality. The important finding of this study was that it is the reason for transfusion rather than the transfusion itself that increased the perioperative risk. Although an association between erythrocyte transfusion and increased perioperative mortality is usually observed, unmeasured confounding factors may also excessively influence this association. One of these unmeasured confounding factors might be the indication for transfusion and might, therefore, directly influence increased perioperative morbidity and mortality itself. This remotely drawn conclusion is extensively discussed in the limitation section of the presented study. However, the relationship between the indication for transfusion and postoperative morbidity and mortality should be interpreted with caution. Due to the retrospective design of the study, important variables that have a major impact on postoperative morbidity and mortality may not have …