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Critical Care  2013 

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

DOI: 10.1186/cc11879

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Abstract:

Given the lack of transfusion trigger and a target haemoglobin concentration post-transfusion in this study, and a generous group upper limit of 90 g/L, would those patients with the lowest initial haemoglobin concentration 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 satisfied of the validity of the multivariate analysis.Donald EG Griesdale, Mypinder S Sekhon, Nielson McLean, William R Henderson and Dean R ChittockWe thank Dr Donnelly for his careful critique of our manuscript [1]. We agree with his assertion that hemoglobin concentration and mortality may be confounded by the severity of illness. In the manuscript, we devote considerable time discussing bias in the context of our results. Despite these concerns, the baseline patient characteristics were remarkably similar between exposure levels. Thus, measured baseline characteristics would be unlikely to account solely for the observed mortality in our study. Although as mentioned in our discussion, we would be unable to account for unmeasured covariates, which could be a significant source of confounding.Another concern expressed by Dr Donnelly was the details of the model building. All covariates included in the final model were chosen a priori based on subject matter knowledge [2]. Age and admission Glasgow Coma Scale were included in

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