Spirometer replacement and serial lung function measurements in population studies : results from the SAPALDIA study

The Swiss Cohort Study on Air Pollution and Lung and Heart Disease in Adults (SAPALDIA), a population cohort study, used heated-wire spirometers in 1991 and 2002 and then ultrasonic spirometers in 2010 revealing measurement bias in healthy never smokers. To provide a practical method to control for measurement bias given the replacement of spirometer in long-term population studies, we built spirometer-specific reference equations from healthy never smokers participating in 1991, 2002, and 2010 to derive individualized corrections terms. We compared yearly lung function decline without corrections terms with fixed terms that were obtained from a quasi-experimental study and individualized terms. Compared with baseline reference equations, spirometer-specific reference equations predicted lower lung function. The mean measurement bias increased with age and height. The decline in forced expiratory volume in 1 second during the reference period of 1991-2002 was 31.5 (standard deviation (SD), 28.7) mL/year while, after spirometer replacement, uncorrected, corrected by fixed term, and individualized term, the declines were 47.0 (SD, 30.1), 40.4 (SD, 30.1), and 30.4 (SD, 29.9) mL/year, respectively. In healthy never smokers, ultrasonic spirometers record lower lung function values than heated-wire spirometers. This measurement bias is sizeable enough to be relevant for researchers and clinicians. Future reference equations should account for not only anthropometric variables but also spirometer type. We provide a novel method to address spirometer replacement in cohort studies.