In addition, data on pulmonary function were collected:
FEV1 in litres and FVC in litres. To calculate the predicted
forced expiratory volume percentage (FEV1% predicted),
data on height and weight (according to the local general
practitioner registry) were collected. Co-morbidities were
measured by the Charlson comorbidity index. This
index is a validated method of classifying comorbidity
from medical records and measures 17 conditions. The
index score is the total of assigned weights and represents a measure of the burden of comorbidity.
Sociodemographic variables, such as age, gender, education level (based on the International Standard Classification of Education: ISCED), working status,
marital status and disease-related variables (medication
use, smoking status) were also collected.