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I read with interest the paper by Sekhon and colleagues, ‘Association of hemoglobin concentration in critically ill patients with severe traumatic brain injury’ (July edition of Critical Care) [1]. Whilst the conclusion (‘mean 7-day haemoglobin concentration <90 g/L associated with increased hospital mortality in patients with severe traumatic brain injury’) is striking, it is plausible, if not likely, that both the haemoglobin defi cit and the observed mortality are dependent upon the severity of the initial injury, and therefore an association between these two might be expected. Given the lack of transfusion trigger and a target haemo globin concentration post-transfusion in this study, and a generous group upper limit of 90 g/L, would those patients with the lowest initial haemoglobin concen tration not be likely to stay below that threshold despite transfusion? Are not the most severe injuries also likely to be associated with a 7-day mean haemoglobin of less than 90 g/L? Injury severity was reportedly accounted for in the multivariate analysis, although the details of the model used were not given. What weighting was given to each variable, such as Glasgow Coma Scale, or CT appearance, and on what basis? Of note is the lack of information regarding other injuries, not just in the multivariate analysis but indeed the entire paper. Were only isolated head injuries considered or were polytrauma patients included in this study? To accept that more than an association exists between two variables - as implied - we must be satisfi ed of the validity of the multivariate analysis.

An expected association between low haemoglobin and poor outcome in severe head injury and an insufficiently detailed multivariate analysis