Table 1.

Summary of Studies and Extracted Study Variables

First author, Year	SubADHD Definition	Criteria	N =	Mean Age	Control? Y/N	Study Length Period	Prevalence Rate of SubADHD	Main Findings and Comments
Epidemiologic / Cross-Sectional
Scahill et al. 1999	Score of 13–18 on ADHD module of DISC-R	DSM-III-R	N = 449	9.2	Y	N/A	22% (n = 100/449) of subjects met criteria for subADHD	Male sex and family dysfunction were significantly associated with subADHD vs. conrols. SubADHD subjects were similarly likely to have a comorbid disorder as the full ADHD group and both differed from controls. SubADHD and fullADHD had greater cognitive impairment than controls, and the full ADHD group was the most severe. Full ADHD group had significantly higher rates of comorbid externalizing disorders (43%) compared to subADHD (16%).
Lewinsohn et al., 2004	≥5 total symptoms	DSM-III-R	N = 1704	16.6	Y	N/A	6% (n=102/1704) of subjects met subADHD criteria	Children with subADHD and full ADHD were more likely to be male vs controls. SubADHD was highly comorbid with other subthreshold disorders, specifically subthreshold MDD, bipolar, eating disorder, anxiety, alcohol and substance abuse/dependence, and conduct. SubADHD was also highly comorbid with the following full-scale disorders: MDD, alcohol abuse/dependence, and conduct.
Cho et al., 2009	≥ 3 ≤ 5 inattentive and/or hyperactive/impulsive symptoms	DSM-IV	N = 1651	9	Y	N/A	9% (n=148/1651) of subjects met subADHD criteria	For temperament profiles, subADHD and full ADHD were associated with significantly higher rates of novelty seeking and lower rates of persistence and self-directedness vs. controls. Children with subADHD were at increased risk for externalizing disorders and impaired scores in most individual and aggregate CBCL scales vs. controls, but the full ADHD group was most impaired. SubADHD subjects had 5x greater risk of having externalizing comorbid disorders (Conduct and ODD) vs. controls. The rate of externalizing and internalizing disorders was significantly lower for subADHD group compared to full ADHD. Both subADHD and full ADHD subjects had significantly lower SES vs. controls. The full ADHD group had higher male preponderance than the subADHD group.
Kim et al., 2009	≥ 3 ≤ 5 inattentive and/or hyperactive/impulsive symptoms	DSM-IV	N = 2673	10.7	Y	N/A	9% (n=231/2673) of subjects met criteria for subADHD	SubADHD subjects had significantly elevated rates of comorbid anxiety, mood, ODD, conduct, tic disorders, and enuresis vs. controls. SubADHD group had similarly high rates of psychiatric comorbidity as full ADHD group, except for anxiety. Having a non-biological primary caregiver, paternal job loss, and parental marital discord were signiciantly associated with subADHD. Male sex and maternal stress during pregnancy were the main predictors of subADHD vs. controls. Maternal stress, depression and alcohol use during pregnancy were all significantly related to full ADHD. Divorce and changes in caregivers were significantly more common in full ADHD than in subADHD. Subjects with subADHD were more likely to have a higher SES compared to full ADHD subjects. Boys were 4 times more likely to have full ADHD and 1.2 times more likely to have subADHD.
Malmberg et al., 2012	Symptoms slightly below threshold based on K-SADS PL (symptoms marked as either ‘possible’ or ‘certain’)	DSM-IV-TR	N = 312	16	N	N/A	47% (n=146/312) of subjects met subADHD criteria	Twin study (156 twin pairs) analyzing subADHD combined type. SubADHD Combined was identified in close to 10% of the sample (n=29/312). SubADHD combined was associated with symptoms of depression, mania, psychosis, anxiety, eating problems, and trauma. In boys subADHD was associated with depression and PTSD.
Larrson et al., 2012	6–7.5 points on the Autism-Tics, ADHD, and other Comorbidities Inventory (A-TAC)	DSM-IV	N = 16,366	9–12	N	N/A	10% (n=1590/16366) of subjects met subADHD criteria	Findings suggest a strong genetic link between the extreme and the subthreshold range of DSM-IV ADHD symptoms. Study suggests that ADHD is an extreme of a continuous trait.
Hong et al., 2014	≥ 3 ≤ 5 inattentive and/or hyperactive/impulsive symptoms	DSM-IV	N = 912	9	Y	N/A	12% (n= 106/912) of subjects met subADHD criteria	SubADHD and full ADHD were both significantly associated with higher ADHD-RS, and more impaired CBCL and Learning Disability Evaluation Scale (LDES) scores vs. controls, although the full ADHD group was significantly impaired vs. both controls and subADHD. Full and SubADHD showed significantly more impaired Stroop scores vs. controls. Full ADHD cases had significantly higher rates of a positive family history of ADHD compared to both subADHD and controls. The rates of Full ADHD (5.2%) and subADHD (16.6%) were higher in children with lower family income vs. children with higher family income.
Roberts et al., 2015	3 inattentive or hyperactive/impulsive symptoms	DSM-IV	N = 4175	11–17	Y	N/A	13% (n = 537/4174) of subjects met subADHD criteria	Within the sub ADHD group, the percentage of subADHD cases that reported impairment was 7.3%. The full ADHD cases had significantly higher self-reported impairment than subthreshold ADHD cases. No other information was provided.
Clinically Referred / Cross-Sectional
DeBono et al., 2012	Symptoms below threshold on K-SADS-PL, no demonstrated evidence of symptomatology across settings, or insufficient impairment to warrant diagnosis	DSM-IV	N = 97	15.57	N	N/A	27.8% (n=27/97) of subjects met subADHD criteria	Both full ADHD and subADHD groups performed in the average range on all written expression measures (spelling, writing fluency, contextual language, and story construction), except for contextual conventions, which was in the low average range for both groups. Processing speed index also fell in the low average range for both groups. No significant differences were found in age, FSIQ (WAIS-III), cognitive processing (WISC-IV, TOWRE, MAND, TROG-2, TLC-EE), and written expression (WJ-III and TOWL-3) between groups. Proportions of learning disabilities did not differ by diagnostic status (18/70 full ADHD and 6/27 subADHD). Parents and teachers rated inattention higher in full ADHD than subADHD (SWAN). Only parents rated hyperactivity/impulsivity higher in full ADHD than subADHD, not teachers (SWAN). No significant group differences were obtained between ADHD subtypes.
Biederman et al., 2018	Age of onset was ≥ 7 years. Had > 5 < 8 ADHD symptoms using DSM-III-R OR ≥ 4 < 6 ADHD inattentive or hyperactive/impulsive symptoms using DSM-IV	DSM-III-R and DSM-IV	N = 2173	11	Y	N/A	6% (n=140/2173) met subADHD criteria	Almost half of subADHD had an age onset of >7 years old and 73% had insufficient symptoms (<8 symptoms of DSM-III-R and <6 of DSM-IV). Both ADHD groups had significantly higher rates of all disorders vs. controls. SubADHD differed from controls in the mean number of comorbid disorders, rates of mood, anxiety and elimination disorders, and substance use disorders, all CBCL clinical and social functioning scales, more than half of SAICA scales, rates of requiring extra help in school and being placed in a special class, having lower scores on WISC-R subscales (excluding Digit Span) as well as in Freedom from Distractibility. SubADHD and full ADHD subjects had similarly elevated GAF scores vs controls. SubADHD had fewer perinatal complications, better family functioning scores, and were more likely to be female, older, and to come from higher SES families.
First author, Year	SubADHD Definition	Criteria	N =	Initial Age – Final Age	Control? Y/N	Study Length Period	Prevalence Rate of SubADHD	Main Findings and Comments
Epidemiologic / Longitudinal
Kadesjö & Gillberg, 2001	5 ≤ 7 of the 14 DSM-III-R symptoms	DSM-III-R	N = 409	7 – 11	Y	4 years	10% (n = 42/409) of subjects met subADHD criteria	Children with subthreshold ADHD had much higher rates of comorbid diagnoses, including externalizing, learning, and developmental disorders compared to controls. The rate of school adjustment, learning, and behavior problems at follow-up was very high in both ADHD groups.
Shankman et al., 2009	≥5 total symptoms	DSM-III-R and DSM-IV	N = 1505	16.6 – 30.4	N	15 years	6% (n=86/1505) of subjects met subADHD criteria.	72% of subADHD subjects had at least one comorbid subthreshold disorder and 36% had a comorbid full syndrome disorder. Although none converted to full ADHD during the 15-year follow-up period, subjects with subADHD diagnosis at baseline were likely to develop alcohol, substance, and conduct disorder at follow up. Males were more likely to have subADHD (64%) than females.
Bussing et al., 2010	≥ 3 ≤ 5 inattentive and/or hyperactive/impulsive symptoms	DSM-IV	N = 332	5 to 11 – 13 to 19	Y	8 years	23% (n=75/332) of youth identified at “high risk for ADHD” met subADHD criteria during childhood	SubADHD was associated with similarly greater risk for grade retention and graduation failure as full ADHD relative to controls. Both full and subADHD groups had lower grades and lower achievement scores on standardized tests of reading and math. Full ADHD and subADHD diagnoses tripled the odds of juvenile justice involvement and the odds of developing ODD, depression and anxiety vs. controls.
Fergusson et al., 2010	≥1 ADHD symptom met on DSM criteria	DSM-IV	N = 995	14 to 16 – 18 to 25	Y	11 years	61% (n=609/995) of subjects met subADHD criteria	In all ADHD cases, clinical subjects had the worst outcomes, those with subclinical symptoms had intermediate outcomes, and controls had the best outcomes. SubADHD subjects were more likely to commit crime (property and violent crime) compared to controls and less likely compared to full ADHD subjects. SubADHD subjects were more likely to develop substance use and mental health issues (major depression, anxiety, antisocial personality disorder, suicide attempts) compared to controls and less likely compared to full ADHD subjects. SubADHD subjects were also more likely to have issues with pregnancy, parenthood, partnership, education, employment, and lower average income compared to controls and less likely compared to full ADHD subjects.
Selinus et al., 2016	6–7.5 points on the Autism-Tics, ADHD, and other Comorbidities Inventory (A-TAC)	DSM-IV	N = 4635	9 or 12 – 15 or 18	Y	6 years	Rate of subADHD for 9 yo twins was 4% (102/2640); Rate of subADHD for 12 yo twins was also 4% (81/1987)	In both the full ADHD and subADHD groups for both twin samples, severity of ADHD symptoms was associated with a higher likelihood for all adverse outcomes (school problems, internalizing, antisocial behavior) in both males and females. Females with subADHD had higher rates of internalizing problems (anxiety, depression, psychosomatic symptoms) vs. controls and vs. the full ADHD group. More girls than boys reported internalizing problems (all levels) and risky drug use (screen-intermediate and screen- positive only). More boys reported antisocial behavior at the screen- negative and screen-intermediate levels, but at the screen-positive level, similar proportions of girls and boys displayed antisocial behavior.