New concepts for expressing forced expiratory volume in 1 s arising from survival analysis

Spirometric lung function is partly determined by sex, age and height (Ht). Commonly, lung function is expressed as a percentage of the predicted value (PP) in order to account for these effects. Since the PP method retains sex, age and Ht bias, forced expiratory volume in 1 s (FEV1) standardised by powers of Ht and by a new sex-specific lower limit (FEV1 quotient (FEV1Q)) were investigated to determine which method best predicted all-cause mortality in >26,967 patients and normal subjects. On multivariate analysis, FEV1Q was the best predictor, with a hazard ratio for the worst decile of 6.9 compared to 4.1 for FEV1PP. On univariate analysis, the hazard ratios were 18.8 compared to 6.1, respectively; FEV1.Ht(-3) was the next-best predictor of survival. Median survival was calculated for simple cut-off values of FEV1Q and FEV1.Ht(-3). These survival curves were accurately fitted (r(2)=1.0) by both FEV1Q and FEV1.Ht(-3) values expressed polynomially, and so an individual's test result could be used to estimate survival (with SD for median survival of 0.22 and 0.61 yrs, respectively). It is concluded that lung function impairment should be expressed in a new way, here termed the FEV1Q, or, alternatively, as FEV1.Ht(-3), since these indices best relate spirometric lung function to all-cause mortality and survival.