The diagnosis of COPD is supported when there is a decrease in:

• A. Functional residual capacity
• B. Residual volume
• C. FEV1/FVC ratio
• D. Total lung capacity

Answer: (C)

Chronic airway obstruction lowers the speed of expiration relative to the total exhaled volume, so the FEV1/FVC ratio falls on spirometry. In COPD, after bronchodilator testing, a reduced FEV1/FVC ratio (commonly <0.70) indicates persistent airflow limitation and supports the diagnosis. Other lung volumes in COPD more often increase due to air trapping and hyperinflation (functional residual capacity, residual volume, and sometimes total lung capacity), rather than decrease, so the ratio is the key decrease that points to COPD.