Abnormal RV/TLC ratio Decreased RV/TLC ratio 2021-05-01T15:44:00Z An abnormally low ratio of residual volume (RV) to total lung capacity (TLC) on pulmonary function testing. RV is the amount of air remaining after maximal expiration and TLC is the total amount of air in the lungs at full inspiration. These volumes cannot be determined by spirometry, but can be measured by inert gas dilution, nitrogen washout, and body plethysmography.