To estimate the effect of using pre- instead of post-bronchodilator lung function on our results, we examined the top set of genome-wide significant results in our largest cohorts with both pre- and post-bronchodilator data and densely imputed genotypes (COPDGene NHW and AA, ECLIPSE, NETT-NAS, and Norway/GenKOLS). Since subjects from these cohorts (except for COPDGene) were included based on post-bronchodilator values, including all subjects with COPD based on post-bronchodilator spirometry would lead to larger sample sizes and make comparison of P-values more difficult. Thus, we chose a random sample of post-bronchodilator cases and controls that matched the number of pre-bronchodilator cases and controls. We performed logistic regression using these equal sized set of pre- and post-bronchodilator cohorts, and meta-analyzed the results.
