We read with interest the paper by Soler-Cataluña and colleagues¹ that examined—in an impressive prospective study with 5 years follow up—factors predicting poor prognosis and mortality in patients with severe acute exacerbations of chronic obstructive pulmonary disease (AECOPD). Their findings are complementary with the current available literature in identifying that older age, arterial carbon dioxide tension, and acute exacerbations were independent predictors of mortality in their cohort group.

We have concerns, however, regarding both their analyses and conclusions. Firstly, several studies^2–,⁴ have given advice on the limitations of dichotomising continuous predictors as they come at a cost “as explanatory variables could be seriously misleading, both in respect of which variables are significant in the model, and perhaps also with respect to the overall predictive ability”.² Soler-Cataluña and colleagues state that in their multivariate model “the frequency of acute exacerbations, age and Charlson index were analysed as categorical variables”.¹

Secondly, and perhaps more importantly, the authors have reported older age (clearly a non-modifiable factor) as a …