Best lung function equations for the very elderly selected by survival analysis

Abstract

We evaluated which equations best predicted the lung function of a cohort of nonagenarians based on which best accounted for subsequent survival. In 1998, we measured lung function, grip strength and dementia score (Mini Mental State Examination (MMSE)) in a population-based sample of 2262 Danes born in 1905. Mortality was registered to 2011 when only five (0.2%) subjects were alive. In half the cohort, we recorded forced expiratory volume in 1 s (FEV1). Complete data were available in 592 subjects with results expressed as standardised residuals (SR) using various prediction equations. Cox proportional hazard regression found lower FEV1SR was a predictor of mortality having controlled for MMSE, grip strength and sex. The US National Health and Nutrition Examination Survey (NHANES) III (1999) equations gave a better spread of median survival by FEV1SR quartile: 3.94, 3.65, 3.51 and 2.61 years with a hazard ratio for death of 1, 1.16, 1.32 and 1.60 respectively, compared with equations derived with the inclusion of elderly subjects. We conclude that extrapolating from NHANES III equations to predict lung function in nonagenarians gave better survival predictions from spirometry than when employing equations derived using very elderly subjects with possible selection bias. These findings can help inform how future lung function equations for the elderly are derived.

FIGURE 1

Kaplan–Meier plots of the proportion of subjects surviving to a given age by sex.

FIGURE 2

A plot of age against the predicted values for the average height and weight of our subjects for each of the equations. NHANES: National Health and Nutrition Examination Survey; ECSC: European Coal and Steel Community; GLI: Global Lung Initiative.

FIGURE 3

Distribution of forced expiratory volume in 1 s (FEV₁) expressed as standardised residuals (SRs) from the predictions using the a) Enright et al. [18], b) National Health and Nutrition Examination Survey (NHANES) III [17], c) European Coal and Steel Community (ECSC) [16], d) Miller et al. [19], e) Global Lung Initiative (GLI) [21] and f) Garcia-Rio et al. [20] equations.