Table 1.

Study population characteristics

No.	Study	Country	Total of samples	Agec		Diagnostic tool
				ADHD	Control
1	Aarts et al. [12]	Netherlands	19 ADHD, 77 controlsa	19.5	27.1	K-SADS based on DSM-IV
2	Jiang et al. [9]	China (Zhejiang)	51 ADHD, 32 controls	8.47	8.5	K-SADS-PL based on DSM-IV
3	Prehn-Kristensen et al. [14]	Germany	14 ADHDb, 17 controlsb	11.9	13.1	K-SADS-PL (Germany translation) based on DSM-IV
4	Wang et al. [10]	China (Taiwan)	30 ADHD, 30 controls	8.4	9.3	K-SADS-E (China version) based on DSM-IV-TR
5	Wan et al. [11]	China (Beijing)	17 ADHD, 17 controls	8 (median)	8 (median)	K-SADS based on DSM-5
6	Szopinska-Tokov et al. [13]	Netherlands	41 ADHD, 15 subthreshold ADHD, 47 controls	ADHD = 20,2; subthreshold ADHD = 20,2	20.5	K-SADS based on DSM-IV

^aControl samples consists of 17 healthy participants, 21 healthy siblings of ADHD patients, and 39 self-reported healthy participants diagnosed by Brain Imaging Genetics (BIG)

^bAll the samples are from male participants. ADHD samples consists of 12 participants with combination type and 2 participants with inattention; 6 patients out of 14 have ODD comorbidities; and 10 patients had been consuming ADHD medication for more than a year to treat ADHD (nine of them had agreed to stop the consumption 48 h prior to samples collection)

^cThe data are displayed in mean