Spirometry measures how much air can be exhaled in one second (forced exhalation volume or FEV1) and the total air that can be exhaled forcefully in one breath (forced vital capacity or FVC).

Restrictive lung diseases, such as scarring due to asbestosis, can reduce the FVC value. Reduced FEV1 or FEV1/FVC values occur due to decreased airflow which may be due to obstructive lung diseases such as COPD or asthma.[13]

COPD usually has a FEV1/FVC ratio below 0.7 and may be confirmatory if this value occurs after using a bronchodilator, indicating a non-fully reversible airflow obstruction. However, routine testing of airways obstruction reversibility in diagnosis may be unhelpful as readings can be inconsistent or subject to spontaneous fluctuations. Nor is it useful as a predictor of the response to long-term therapy.[4]

Spirometry should be used to monitor disease progression or if there is an exceptionally good response to treatment, which may suggest a COPD diagnosis needs reconsideration. If asthma is still a possible diagnosis, the patient should be asked to do a series of peak flow measurements at home.[4]