Lung function measurement
In a patient with a clinical history and symptoms suggestive of COPD, measure lung function with spirometry to confirm the diagnosis. COPD cannot be diagnosed without spirometry.
A postbronchodilator forced expiratory volume in 1 second to forced vital capacity ratio (FEV1/FVC ratio) less than 0.7 is diagnostic of COPD. This is a widely accepted practical definition.
A chest X-ray can be useful to exclude other causes of breathlessness. A computed tomography (CT) scan is not required for diagnosis of COPD.
More extensive lung function testing (eg plethysmographic lung volumes) will often also show gas trapping (increased residual volume) and a diffusion defect (decreased diffusing capacity for carbon monoxide [DLCO]). However, these tests are not routinely required as part of the diagnosis of COPD.
See Pulmonary function testing for detail on the measurement and interpretation of pulmonary function tests.