Table 3.

Subgroup analysis to assess the serum and hair zinc levels in subjects with ADHD.

Sub grouped by	No.	WMD (95% CI)	P value	P-Heterogeneity	I2 (%)	P-between subgroup heterogeneity
Serum zinc
Sample type
Serum	10	− 0.67 (− 4.23, 2.90)	0.714	< 0.001	98.7
Plasma	3	− 1.73 (− 3.49, 0.02)	0.053	< 0.001	93.2	0.192
Blood	1	− 3.24 (− 4.95, − 1.53)	< 0.001	0	0
Publication year
≤ 2010	4	− 2.92 (− 5.85, 0.02)	0.052	< 0.001	95.8	< 0.001
> 2010	10	− 0.48 (− 3.05, 2.09)	0.715	< 0.001	98.1
Zinc assessment method
Atomic absorption spectrophotometer	6	− 2.79 (− 5.04, − 0.53)	0.015	< 0.001	97.3	< 0.001
Other	8	0.23 (− 3.82, 4.28)	0.912	< 0.001	98.4
Sample size
≤ 100	11	− 0.88 (− 4.14, 2.39)	0.598	< 0.001	98.6	0.004
> 100	3	− 1.49 (− 2.32, − 0.67)	< 0.001	0.066	63.1
Study design
Case–control	13	− 2.31 (− 3.97, − 0.64)	0.007	< 0.001	96.7	< 0.001
Cross-sectional	1	14.14 (12.52, 15.76)	< 0.001	< 0.001	0
Hair zinc
Publication year
≤ 2010	1	− 65.10 (− 191.40, 61.20)	0.312	0	0	0.237
> 2010	7	− 4.54 (− 38.76, 29.68)	0.795	< 0.001	98.2
Zinc assessment method
Atomic absorption spectrophotometer	1	97.33 (33.73, 160.93)	0.003	0	0	0.008
Other	7	− 19.9 (− 54.14, 15.95)	0.286	< 0.001	98.2
Sample size
≤ 100	5	− 11.74 (− 49.82, 26.33)	0.545	< 0.001	98.8	0.712
> 100	3	− 2.78 (− 119.62, 114.07)	0.963	0.003	82.8
Study design
Case–control	7	− 11.79 (− 49.59, 26.01)	0.541	< 0.001	98.3	0.544
Cross-sectional	1	17.57 (− 3.18, 38.32)	0.097	0	0