Table 2.

Smoking amounts of COPDs and controls in the included studies.

Study and reference	Pack-years of smoking
	Controls (N)	COPDs (N)
Bergen cohort from Pillai et al [26]	19 ± 13*(810)	32 ± 19*(823)
Copenhagen City Heart Study from Kaur-Knudsen et al [25]	NR	NR
COPDGene study from Kim et al [24]	52 ± 29*(335)	37 ± 19*(507)
COPACETIC cohort from Lambrechts et al [23]	38.0 (28.0-46.2) (295)	38.7 (29.7-49.5)(161)
LEUVEN cohort from Lambrechts et al [25]	42.3 (30.0-50.5) (184)	40.0 (30.0-57.0) (475)
NETT/NAS cohorts from Pillai et al [26]	40 ± 28*(472)	66 ± 30*(389)
New Zealand study from Young et al [27]	40 ± 19*(488)	47 ± 20*(458)

Abbreviations: Data were presented as median (25th - 75th percentiles) for COPACETIC and LEUVEN cohorts, and as mean ± SD for other studies. NR, not reported; COPD, chronic obstructive pulmonary disease, defined by spirometric results; * p < 0.05