Experience in a clinical trial for COPD suggests the reduced back-extrapolated volume limit of 0.100 L may be unnecessarily restrictive

<bold>Background:</bold> Excessive hesitation at the start of a forced exhalation can result in an artifactually high FEV1 because of a delayed dynamic airway compression. The back-extrapolated volume (BEV) limit of 5% of the FVC was not changed in the 2019 ATS/ERS standards but the volume arm of the standard that applies when the FVC is < 2.00 L was reduced from 0.150L to 0.100L. <bold>Objective:</bold> To evaluate the impact of the 2019 ATS/ERS BEV volume limit on FEV₁ accuracy. <bold>Methods:</bold> We compared FEV1s in 959 measurements from 326 subjects showing the FVC < 2.00L in a clinical trial for COPD. 57 of the 959 measurements showed some efforts in a measurement meeting the 2019 BEV limit of 0.100 L and other efforts showing BEV > 0.100 but < 0.150 L, the BEV limit from the 2005 standards. The reported FEV₁ from the measurements showing BEV < 0.100L was compared to the highest FEV1 from the efforts showing BEV > 0.100 L but < 0.150 L. <bold>Results:</bold> The mean difference between the reported FEV₁ (BEV < 0.100 L) and the largest FEV₁ from an effort showing a BEV > 0.100 L but < 0.150 L was 0.006 L <span>+ 0.065 L. Fifty five of the 57 comparisons showed the difference to be within the expected repeatability agreement of the two largest FEV₁s within a measurement (0.150 L). <bold>Conclusions:</bold> The difference between the ATS 2019-compliant FEV₁ and the largest FEV₁ from an effort showing a BEV between 0.100 L and 0.150 L was not statistically or clinically significant (p = 0.469). The maximum allowable BEV volume of 0.100 L, compared to the limit of 0.150 L does not appear to provide a significant improvement in FEV₁ accuracy in measurements from a cohort of COPD patients. Additional evaluation is required.