Minimal clinically important difference--exacerbations of COPD

Exacerbations of COPD are now recognised as being important events in the natural history of the condition and become more frequent as the disease worsens. Defining a minimum clinically important difference in exacerbation rate is fraught with difficulty. There is substantial between and within subject differences in the occurrence of these events that makes an individual evaluation of their importance problematic. At present, the most widely used definition of an exacerbation identifies an episode where the patient seeks medical help rather than a predefined change in one or more symptoms. Despite these problems, intervention studies with bronchodilator drugs, inhaled corticosteroids, and pulmonary rehabilitation appear to reduce the frequency of exacerbation events. In patients with an FEV1 below 50% predicted there is reasonable consistency about the magnitude of change and a 4-unit improvement in the St George's Respiratory Questionnaire is commonly associated with a 20-25% reduction in the reported number of exacerbations. Individual studies vary depending upon the recruitment protocol. Patients who experience symptomatic benefit may be those in whom a clinically important change in exacerbations occurs but this concept requires testing prospectively. Existing methodologies for estimating clinically important differences are hard to apply with a binary outcome like this, and more work will be needed to develop a robust approach for dealing with this important clinical variable.