Clusters, classification and epidemiology of interstitial lung diseases: concepts, methods and critical reflections.

The present article reports on two conceptual and methodological issues concerning interstitial lung disease (ILD) about which there is a lot of misunderstanding and contradiction: the investigation of epidemics and clusters and the classification and epidemiology of ILD. In general, an epidemic is the occurrence of cases of an illness in excess of normal expectancy. The investigation of an epidemic often demands a number of sequential studies: first descriptive, then aetiological. Clusters consist of an increase in incidence of much smaller magnitude, perhaps excluding the possibility that this is merely a result of chance. In recent years, more valid statistical methods for the assessment of clusters have developed. For interstitial lung disease in particular, different classifications exist that are sometimes inconsistent and may change with time. All diseases require a process of ascertainment, whereby they are identified, classified and perhaps registered. The two most employed epidemiological techniques for testing aetiological hypotheses are the cohort and case-referent approaches.