The FEV₁ quotient (FEV₁Q), calculated as the index between FEV₁ and a theoretical lower survivable FEV₁ threshold of 0.4L for females and 0.5L for males, has been investigated as a novel method of interpreting results from lung function testing. The applicability of the FEV₁Q in populations with chronic respiratory failure has not been studied, and the continuous association between the FEV₁Q and mortality is unknown.

Longitudinal analysis of data from the DISCOVERY database. First percentile values of FEV₁ were determined. The predictive ability of FEV₁Q and FEV₁ %-predicted values for overall and respiratory mortality were compared using Cox and Fine-Gray regression models with C-statistics. The continuous association between FEV₁Q and mortality was evaluated using a restricted cubic spline.

A total of 5,711 patients (61 % females) with oxygen-dependent COPD were studied. First-percentile values of FEV₁ were 0.3L for females, and 0.4L for males. Higher levels of FEV₁Q were associated with a lower risk of overall and respiratory mortality when adjusting for age, sex, height, smoking status, A-a-gradient, and education. For overall mortality, FEV₁Q and FEV1 %-predicted models had identical C-statistics of 0.60 (95 %CI 0.59–0.61). The association between FEV₁Q and overall mortality was J-shaped, with a threshold of increased risk at FEV₁Q values < 1.0.

While first-percentile values of FEV₁ were lower in this cohort than in previous studies, a population threshold for increased mortality risk was identified at FEV₁Q levels corresponding to those originally presented. For individual subjects, neither FEV₁Q, nor FEV1 %-predicted, were identified as useful predictors of mortality.