Introduction: Static lung volume testing is needed for the classification of physiologic impairment (Eur Respir J 2022; 60:2101499). Measurement of lung volumes by different methods are not interchangeable.

Objectives: We aimed to quantify differences in total lung capacity (TLC) in normal and COPD subjects using three different methods.

Methods: We performed spirometry and lung volume testing in normal and COPD subjects in the same PFT session. TLC was measured using body plethysmography (TLCpleth), multiple-breath nitrogen washout (TLCN₂) and single-breath gas dilution (TLCsb) methods. TLCsb was calculated as alveolar volume (VA) + 150 mL. Testing was performed in accordance with ERS/ATS standards and only acceptable and repeatable tests were included. A paired samples t-test was applied to determine significance between TLC measurements.

Results: We tested 11 normal and 5 COPD subjects. Mean FEV₁ percent predicted for COPD subjects was 59%. TLC and spirometry measurements are listed in the table. Significant differences were found between TLCpleth and TLCN₂ (p<0.01) and TLCsb (p<0.01) for both groups.

Conclusions: TLC measurement was significantly different using the three methods. TLCpleth was 0.46 L larger than TLCN₂ and 0.61 L larger than TLCsb in normal subjects; corresponding differences in COPD subjects were 1.17 L and 1.50 L. These differences need to be considered when determining presence or severity of physiologic impairment.