Respiratory mortality in chronic obstructive pulmonary disease: fixed ratio versus lower limit of normal in smokers

Considerable debate persists regarding the criteria for spirometric airflow obstruction for the diagnosis of chronic obstructive pulmonary disease (COPD).1 Although several recent studies indicate that the fixed threshold of ratio of the forced expiratory volume in the first second to the forced vital capacity (FEV1/FVC) of 0.70 is associated with COPD-related clinical outcomes with a higher discriminative accuracy than the statistical distribution-based lower limit of normal (LLN),2-4 the latter has received the imprimatur from respiratory societies for the diagnosis of airflow obstruction.5 The main controversy involves individuals whose FEV1/FVC falls below the fixed threshold but not below the LLN. Arguments have been made that individuals in this intermediate zone do not have respiratory disease but instead have a low ratio due to older age and therefore do not have COPD. Although epidemiologic studies have demonstrated a lower survival for these individuals compared to individuals without airflow obstruction by either the fixed threshold or LLN,6,7 this higher mortality has been ascribed to the ageing process and the associated cardiovascular disease. The evidence for this assertion, however, remains limited. We, therefore, aimed to evaluate cause-specific mortality in individuals in the intermediate zone.