Pulmonary function testing is an important means of diagnosing COPD and for monitoring its progression. Spirometry is frequently used for recording long-term deterioration of patients through changes in forced expiratory volume in 1 second (FEV1). The annual loss in healthy people is approximately 30ml, this rises to 100ml in severely AAT deficient patients. FEV1 measurements require a lot of cooperation from the patient and a number of attempts may be needed to gather reliable data.
As emphysema progresses the vital capacity (VC) may slowly increase. Whole body plethysmography is required for more precise determination of this. Other tests that may be found to be necessary are for increased airway resistance (Raw) and elevated residual volume (RV).
Measurement of carbon monoxide diffusion capacity (DLCO) is another technique of lung function testing. This shows the global gas exchange potential between the alveolar pace and the pulmonary capillaries. AATD patients have a DLCO of 50 ± 22% of normal values.