Serum was collected at the time subjects were enrolled in the study when they had not experienced an AECOPD for a minimum of 4 weeks and was stored at −80°C until MBL concentrations were assayed by enzyme-linked immunosorbent assay (R & D Systems, Minneapolis, MN). Samples were diluted 1/500 for this assay and assayed in duplicate wells. At this dilution, the range of the standard curve corresponds to concentrations ranging from 78 ng/mL to 5000 ng/mL. When concentrations were extrapolated above 5450 (n = 7) or below 60 (n = 6), the samples were reassayed at either a 1/2500 or a 1/20 dilution and these new values used as MBL serum concentrations. In one case, a sample was still less than the detection limit at 1/20 dilution and this sample was assigned the concentration of less than 3 pg/mL.