Table 1.

Characteristics of each study included in the meta-analysis.

Study	Site	Design	Type of sample	NGS sequencing	Region	n0	n1	n2
Park et al. (8)	Oral	Case-control	Oropharyngeal swab	454 pyrosequencing	V1-V3	12	17	–
Wu et al. (9)	Oral	Case-control	Periodontal plaque	Illumina	V4-V5	25	25	–
Wang et al. (10)	Airway	Case-control	Sputum	Illumina	V4	16	43	–
Haldar et al. (11)	Airway	Case-control	Sputum	Illumina	V4	73	60	–
Wang et al. (12)	Airway	Case-control	Sputum	Illumina	V3-V4	10	4	36
Wang et al. (13)	Airway	Case-control	Sputum	PacBio/Illumina	V1-V9	27	98	–
Mahomed et al. (14)	Airway	Case-control	Sputum	Illumina	V1-V3	–	18	6
Dang et al. (15)	Airway	Case-control	Sputum	NovaSeq-PE250	V3-V4	19	51	–
Su et al. (16)	Airway	Case-control	Sputum	Illumina	V3-V4	10	23	28
Zhu et al. (17)	Airway	Case-control	Sputum	Illumina	V3-V4	36	–	34
Millares et al. (18)	Airway	Cross-sectional	Sputum	454 pyrosequencing	V1-V3	-	8	8
Feigelman et al. (19)	Airway	Case-control	Sputum	Illumina	NR	4	4	–
Erb-Downward et al. (20)	Airway	Case-control	BALF	454 pyrosequencing	V1-V3	10	4	–
Pragman et al. (21)	Airway	Case-control	BALF	454 pyrosequencing	V3	10	22	–
Einarsson et al. (22)	Airway	Case-control	BALF	Illumina	V1-V2	11	18	–
Mika et al. (23)	Airway	Cross-sectional	BALF	454 pyrosequencing	V3-V5	10	32	–
Gupta et al. (24)	Airway	Case-control	BALF	Illumina	V3-V4	-	14	13
Kim et al. (25)	Airway	Case-control	lung tissue	454 pyrosequencing	NR	13	13	–
Wu et al. (26)	Intestine	Case-control	Fecal	Illumina	V3-V4	22	29	29
Bowerman et al. (27)	Intestine	Case-control	Fecal	16S rRNA gene sequencing	NR	29	28	–

NR, not reported; n₀, Healthy; n₁, SCOPD; n₂, AECOPD; “-”, none.