The definition and classification of pneumonia.

Following the publication of a volume of Pneumonia focused on diagnosis, the journal's Editorial Board members debated the definition and classification of pneumonia and came to a consensus on the need to revise both of these. The problem with our current approach to the classification of pneumonia is twofold: (i) it results in widespread empirical, and often unnecessary, use of antimicrobials that contributes to pathogen resistance; and (ii) it contributes to heterogeneity among the groups of subjects compared in research, causing misclassification bias and mixtures of effects that threaten internal validity. After outlining the problem of classification, this commentary describes the strengths and weaknesses of a range of systems for the classification of pneumonia. The commentary then calls for debate to generate consensus classifications in the field, proposing a working definition and way forward focusing on the following three points: (i) pneumonia should be defined as an acute infection of the lung parenchyma by various pathogens, excluding the condition of bronchiolitis; (ii) defining pneumonia as a group of specific (co)infections with different characteristics is an ideal that currently has limited use, because the identification of aetiologic organisms in individuals is often not possible (however, the benefits of classifying pneumonia into specific, more homogenous phenotypes should be carefully considered when designing research studies); and (iii) investigation of more homogenous pneumonia groupings is achievable and is likely to yield more rapid advances in the field.