Table 1 Inclusion criteria for studies

1

Study included COPD patients as a comparator group or a population defined by lung function

2

Participants must be free of malignancies

3

Articles must be original research and have been published as a full-text in a peer-reviewed journal

4

Studies must have been carried out in humans (exclusion of in vivo and in vitro study designs)

5

Studies must assess relative levels of DNA methylation

6

Analysis of DNA methylation must have been in peripheral blood

7

Abstracts must be available in English.