[Methodology and clinical impact of studies on disease progression].

Because of the progressive nature of chronic obstructive pulmonary disease (COPD), interventions designed to halt disease progression are especially important. Two of the most commonly evaluated variables are the yearly decrease in forced expiratory volume in 1 second (FEV(1)) and the number of exacerbations. The effect of inhaled corticosteroids on deterioration of FEV(1) is slight, being limited to a mild transitory improvement. Tiotropium may have a greater effect on the yearly decrease in FEV(1), although more information is required. Both tiotropium and inhaled corticosteroids reduce the number of COPD exacerbations. However, to interpret these results, several methodological limitations need to be assessed, such as the variability and accuracy of FEV(1) measurements, the high withdrawal rate, dependence on baseline function, the effect of steroid suppression, and the computation and analysis of exacerbation rates.