Table 2. Baseline characteristics of studies included in systematic review.

Study	Study design	Sample	Setting/country	Mean/median age	% Male	Covariates	Median follow-up duration (yr)	Hazard ratio (95% CI)	Newcastle-Ottawa scale score
Fang et al. [23] 2015	Retrospective nested case-control	205,266	Administrative database	NR	Not reported	Age, sex, income, region, urbanization, and CCI	11	2.1 (1.16–3.8)	8
			Taiwan
Gao et al. [33] 2020	Mendelian randomization	4,378	Clinic-based	NR	0	Age, sex, smoking status, family history of cancer and BMI	NR	European population: 2.47 (1.86–3.27)	6
			China					Asian population: 1.33 (1.13–1.56)
Justeau et al. [26] 2020	Prospective cohort	8,748	Clinic-based	61	64.5	Age, sex, BMI, smoking status, alcohol intake, diabetes, hypertension, medical history of cardiac disease and Chronic obstructive pulmonary disease, marital status, type of sleep study, and study site	5.8	Mild OSA: 2.04 (1.05–3.98)	9
			France					Moderate OSA: 1.40 (0.69–2.87)
								Severe OSA: 1.14 (0.50–2.58)

CI, confidence interval; CCI, Charlson Comorbidity Index; BMI, body mass index; NR, not reported; OSA, obstructive sleep apnea.