This study has several limitations. Objective measures of
COPD severity were not included because all data were collected via self-report questionnaires. Oxygen use was included in the regression models as a proxy for disease severity, but airflow limitation was not objectively quantified. There was a 36 % initial response rate, and some individuals who returned the questionnaire were excluded from analyses due to missing data. A comparison of individuals who were included to those
who were excluded indicated that the two groups differed with regard to only 2 of the 17 variables used in these analyses. The percentage of individuals who did not know their genotype was higher in the group that was removed from the sample due to missing data. This may reflect an underlying lack of commitment, both to knowing their genotype and to completing a questionnaire focused on their health condition.