Table 2.

Results of subgroup analyses analysis of association studies between peripheral blood glutamate levels and MDD

Variables	Number of association studies	Subgroup analysis				Heterogeneity
		SMD	95% CI lower	95% CI upper	p-value	l2 (%)	Q (p-value)
Blood sampling
Plasma	10	0.62	0.31	0.93	0.0001	75.9	2.9×10−4
Serum	2	0.16	−0.12	0.45	0.26	0	0.42
Glutamate assay
HPLC	8	0.41	0.22	0.60	<0.0001	28.5	0.22
MS	4	0.69	−0.069	1.45	0.075	89.7	<1.0×10−4
Medication
Medication free	8	0.59	0.19	1.00	0.004	72.9	6.8×10−4
Food consumption
Fasting	4	0.33	0.063	0.59	0.015	0	0.51
Not fasting	3	0.90	0.031	1.77	0.042	94.1	<1.0×10−4
Ethnicity
Asian	7	0.58	0.14	1.02	0.01	88.6	<1.0×10−4
Non-Asian	5	0.48	0.21	0.76	0.0006	0	0.84

Abbreviations: MDD, major depressive disorder; SMD, standardized mean difference; HPLC, high-performance liquid chromatography; MS, mass spectroscopy.