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A brief comment about predictive models for mortality in intensive care units

Editor, Granholm and colleagues recently published an interesting paper in the Acta Anaesthesiologica Scandinavica concerning the construction and internal validation of a scoring system for 90day death after admission to the intensive care unit (ICU). Nevertheless, we have a few observations concerning the development of the model used, as well as a comment they make about a similar study undertaken by us. The sample used by Granholm and colleagues to develop their model includes data from different sources, including observational studies and clinical trials. This could, in theory, provide added strength to the results, but only if the different data were shown to be homogenous, which they were not. Indeed, an important proportion of the data are taken from clinical trials (47.7%), which thus prevents any generalization of the results, as the inclusion and exclusion criteria for these trials are very strict and the treatment received could affect the outcome of the model developed, in addition to which the prevalence of sepsis was considerably increased (at least 47.7% = 1947/4086) and this does not correspond to the usual patient population in the ICU. Nor is it known what happened to the patients who survived beyond 90 days. Accordingly, the model developed does not appear to be extrapolatable to general clinical practice in the ICU (non-representative study sample). The authors also mention that our study was undertaken in just one centre. Although we mentioned that this was, to a certain extent, a limitation of our study (requiring external validation), it nevertheless had the advantage that the study sample was completely homogenous. Given the number of events available in the study by Granholm and colleagues (n = 1403), the authors could well have used some of the data to develop the model and validate it internally, and other data to validate it externally, as they would have at least 100 events available for the validation sample. The authors also state that our model is likely underpowered and had a high risk of overfitting. To achieve valid results of the sample size, based on the ratio of events-per-variable (EPV), it should be equal to or greater than ten, as Granholm and colleagues also mention. The EPV in our study was 15. Finally, although the outcomes of the two models are different, they both have clinical usefulness.