Predictors of longitudinal change in diffusing capacity over 8 years

Low single-breath diffusing capacity (DLCO) values are associated with anatomic emphysema, but the predictors of longitudinal change in DLCO over many years are unknown. Study subjects were adult participants in the longitudinal Tucson Epidemiology Study of Obstructive Lung Disease who had at least one DLCO measurement during either of two surveys 8 yr apart (n = 543). Smoking status was determined at each examination (current, former, or never smoker). Quitters were defined as those currently smoking at the baseline DLCO examination (1982-1983) and self-reported as no longer smoking at the follow-up exam (1990-1991). The longitudinal DLCO data were analyzed using repeated measures analysis; because of missing observations this was done using a saturated random effects model. The results showed that males had higher levels of DLCO than females, current smokers had significantly lower levels of DLCO than never smokers, but there was no difference in their mean slopes over time. Smoking history, assessed using pack-years of smoking, was associated with reduced DLCO levels, independent of whether current or ex-smokers. Males and females demonstrated equivalent rates of decline in DLCO that accelerated with increasing age, and mean DLCO declines were associated with declines in FEV1 between surveys.