Some limitations of the present study should be
mentioned. First, pulmonary function testing was
not standardized between hospitals. This may have
biased the results of pulmonary function tests. Second,
the ratio of men to women in the present study
is higher in comparison with Western countries, and
generalization of the results of men to women with
COPD might be questionable. However, this study
population reflects the characteristics of patients
with COPD in Japan, and the gender differences
likely reflect past trends in smoking. Third, the 81%
follow-up rate was somewhat lower than typical for
this kind of study. In the Japanese health-care
system, patients have free access to health-care
facilities, including hospitals, at any time and may
choose to change facilities, making follow-up potentially
more difficult. However, there were few differences
in baseline characteristics between the dropouts
and the subjects who were successfully followed
up. Therefore, we believe it is unlikely that the high
dropout rate reflects increased patient mortality.