The review started in February 2013, and was designed to identify and update any previous systematic reviews that have assessed whether active smoking can increase the risk of developing COPD. Only one previous systematic review assessing the association between active smoking and COPD was identified (Forey 2011). To update this earlier work and ensure all additional eligible papers are included in the present systemic review, a search for any other original studies was conducted from 1985 onwards.
In total, 24 studies have been included in the present systematic review, which has assessed the effect of active smoking (versus non- or never-smoking) on the diagnosis of COPD. However, only 22 of the studies fulfilled the criteria for inclusion in the meta-analysis.
Twenty-one of the studies used a prospective cohort design, one a retrospective cohort design, and two studies used nested case-control designs.
Thirteen of the included studies used the GOLD criteria to define COPD based on forced expiratory volume in one minute (FEV1)/forced vital capacity (FVC) <70%; however, one of these studies required an extra criterion of FEV1 <80%. The remaining studies used a range of criteria to define COPD.
All studies measured participants’ active smoking through self-report. One study assessed the validity of the self-reported information using biochemical verification.
Fifteen of the studies were conducted in Europe; the remaining studies were conducted in the USA, Japan, Thailand or Australia.
The methodological quality of the 24 studies, as judged by the Newcastle-Ottawa scale score, gave an overall median score of 6 (range 4-8). Using the a priori chosen cut off of 7 to indicate high methodological quality, five (21%) of the studies were judged to be high quality.
Publication bias was assessed visually using a funnel plot for the association between active smoking and the risk of developing COPD; it included data for the 22 studies included in the meta-analysis. There was no clear evidence of publication bias identified from this funnel plot. Click here to see a funnel plot of the findings – Figure A.
For details of all the studies used, please click here (PDF).