Spirometry is the classic pulmonary function test, which measures the volume of air inspired orexpired as a function of time. It can monitor quiet breathing and thereby measure tidal volume, andalso trace deep inspirations and expirations to give information about vital capacity. Spirometrymay also be used to measure forced expiration rates and volumes and to compute FEV1/FVC ratios (seethe encyclopedia page on forced expiration for more information).
Spirometry cannot, however, access information about absolute lung volumes, because it cannot measurethe amount of air in the lung but only the amount entering or leaving. Thus information aboutfunctional residual capacity, and lung volumes computed from FRC, such as total lung capacity andresidual volume, must be computed via different means, such as body plethysmography or gas dilution.