Table 1. Characteristics of the total study population by smoking status.

Variables	Total† (N = 21008)	%	Ever-smoker (n = 5195)	%	Never-smoker (n = 15813)	%	p for χ2 test‡
Gender							<0.001
Male	9724	46.29	5061	97.42	4663	29.49
Female	11284	53.71	134	2.58	11150	70.51
Age (years)							<0.001
5–19	3556	16.93	46	0.89	3510	22.20
20–29	2053	9.77	514	9.89	1539	10.53
30–39	2181	10.38	599	11.53	1582	12.06
40–49	4648	22.12	1304	25.10	3344	21.15
50–59	3612	17.19	1170	22.52	2442	15.44
60–69	3136	14.93	1060	20.40	2076	13.13
≥70	1822	8.67	502	9.66	1320	8.35
Median (inter quartile range)	46 (27–59)		50 (41–62)		44 (23–58)
Highest education level							<0.001
Primary school or lower	11172	53.18	2236	43.04	8936	56.51
Middle school	6882	32.76	2198	42.31	4684	29.62
High school	2301	10.95	650	12.51	1651	10.44
College or higher	653	3.11	111	2.14	542	3.43
BMI (kg/m2)							<0.001
<18.5	3278	15.60	278	5.35	3000	18.97
≥18.5-<24.0	10082	47.99	2828	54.44	7254	45.88
≥24.0-<28.0	5596	26.64	1542	29.68	4054	25.64
≥28.0	2051	9.76	547	10.53	1504	9.51
Household per capita income (RMB)							<0.001
<6000	13411	63.84	3178	61.17	10233	64.72
≥6000	7596	36.16	2017	38.83	5579	35.28
Alcohol drinking							<0.001
No	17017	81.01	2422	46.62	14595	92.30
Yes	3990	18.99	2773	53.38	1217	7.70
TB contact history							0.005
No	20185	96.12	4958	95.47	15227	96.34
Yes	814	3.88	235	4.53	579	3.66
History of T2DM							<0.001
Yes	1017	4.84	350	6.74	667	4.22
No	19911	95.16	4845	93.26	15146	95.78
QFT test							<0.001
Positive	3954	18.82	1468	28.26	2486	15.72
Negative	16454	78.32	3624	69.76	12830	81.14
Indeterminate	600	2.86	103	1.98	497	3.14

Abbreviations: BMI = body mass index; IQR = interquartile range; QFT = QuantiFERON-TB Gold In-Tube; T2DM = type 2 diabetes mellitus; TB = tuberculosis.

^†Sum might not always be in total because of missing data. Frequency of missing data did not differ significantly between smoking statuses.

^‡ P values refer to the comparison of different variables between ever- and never-smokers.