Twenty-Year Trends in Diagnosed Attention-Deficit/Hyperactivity Disorder Among US Children and Adolescents, 1997-2016

IMPORTANCE Attention-deficit/hyperactivity disorder (ADHD) is common in US children and adolescents. It is important to understand the most recent prevalence of ADHD and its long-term trends over the past decades. OBJECTIVE To estimate the prevalence of diagnosed ADHD and 20-year trends from 1997 to 2016 among US children and adolescents using nationally representative data. DESIGN, SETTING, AND PARTICIPANTS In this population-based, cross-sectional survey study (National Health Interview Survey), surveys were conducted annually from 1997 to 2016. A total of 186457 children and adolescents aged 4 to 17 years from 1997 to 2016 were included in this analysis. Data were collected through in-person household interviews with a parent or guardian. The data analysis was performed in January 2018.


Introduction
Attention-deficit/hyperactivity disorder (ADHD) is a childhood-onset neuropsychiatric disorder characterized by persistent and impairing inattention, hyperactivity, and impulsivity. 1,2The symptoms of ADHD often persist into adulthood.Early comorbidities concurrent with ADHD may include tic disorder, anxiety disorder, autism spectrum disorder, communication and specific learning or motor disorders (eg, reading disability, developmental coordination disorder), and intellectual disability. 1 Long-term follow-up studies from childhood to adulthood found that children with ADHD, compared with those without ADHD, were more impaired in psychosocial, educational, and neuropsychological functioning 3 and had higher risks for antisocial disorders, major depression, and anxiety disorders as adults. 4e American Psychiatric Association states in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition that 5% of children have ADHD, based on previous worldwide estimates in earlier years. 5,6The prevalence of ADHD varies across different countries, with a significantly higher prevalence in the United States than in European countries. 7Moreover, the prevalence of ADHD has changed over time.][10][11][12][13][14][15][16][17][18][19] For example, an analysis of the National Health Interview Survey (NHIS) reported a 33% increase in ADHD prevalence from 1997-1999 (5.7%) to 2006-2008 (7.6%)   among children and adolescents aged 3 to 17 years. 8Similarly, the National Survey of Children's Health showed a 42% increase between 2003 and 2011 in the prevalence of diagnosed ADHD among children and adolescents aged 4 to 17 years. 9formation about the current prevalence of ADHD and its long-term trends over the past decades is needed to inform future research, clinical care, and policy decision making on ADHD.Therefore, we analyzed nationally representative data to estimate the most recent prevalence of ADHD diagnosis among US children and adolescents and the 20-year trends from 1997 to 2016.

Study Population
The NHIS is a leading national health survey in the United States. 20It is conducted annually by the National Center for Health Statistics at the Centers for Disease Control and Prevention.The NHIS is composed of a series of nationally representative cross-sectional surveys.With a large sample size and a relatively high response rate, the NHIS has become the principal source of information on the health of the civilian, noninstitutionalized household population of the United States. 21The NHIS survey methodologic reporting is consistent with the reporting standards by the American Association for Public Opinion Research. 22

Data Collection
The NHIS collected data on a variety of health topics through in-person household interviews.For each interviewed family in the household, 1 sample child, if any, was randomly selected by a computer program. 20Information about the sample child was collected by interviewing an adult, usually a parent, who was knowledgeable about the child's health.
From 1997 to 2016, respondents were asked, "Has a doctor or health professional ever told you that [the sample child] had attention-deficit/hyperactivity disorder (ADHD) or attention-deficit

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Twenty-Year Trends in Diagnosed ADHD Among US Children and Adolescents disorder (ADD)?"Approximately 99.8% of the participants responded to this question.The respondents in the 2016 survey were further asked, "Does [the sample child] currently have attention-deficit/hyperactivity disorder (ADHD) or attention-deficit disorder (ADD)?"Demographic data, such as age, sex, race/ethnicity, family income, and geographic region, were collected using a standardized questionnaire during the interview.Race and Hispanic ethnicity were self-reported and classified based on the 1997 Office of Management and Budget Standards.Family income levels were classified according to the ratio of family income to federal poverty level lower than 1.0, 1.0 to 1.9, 2.0 to 3.9, and 4.0 or higher.Data analysis was performed in January 2018.

Statistical Analysis
All eligible children and adolescents aged 4 to 17 years who participated in the NHIS were included in the present study.Only 0.2% of participants had missing information on ADHD diagnosis and were therefore excluded.We restricted the age range to 4 to 17 years because the clinical guidelines of the American Academy of Pediatrics recommended to initiate an evaluation for ADHD in children aged 4 years or older. 23 estimated the prevalence estimates with survey weights to account for unequal probabilities of selection, oversampling, and nonresponse in the survey.We also used survey design variables about strata and primary sampling units for each survey cycle.In this study, we combined each 2-year period (eg, 1997 and 1998) as 1 survey cycle (eg, cycle 1997-1998).P values for overall differences across strata were calculated using χ 2 tests.Trends in the prevalence over time were tested using a weighted logistic regression model, which included survey cycle as a continuous variable and adjusted for age, sex, and race/ethnicity.To determine whether the secular trends differ across strata, interaction analyses were performed by including multiplicative terms of each strata variable with survey cycle in the aforementioned logistic regression models.All analyses were conducted using survey procedures in SAS, version 9.4 (SAS Institute).Two-sided P < .05 was considered statistically significant.

Prevalence of Diagnosed ADHD Among US Children and Adolescents in 2015-2016
Among the included 18   1).

Trends in Diagnosed ADHD in US Children and Adolescents From 1997 to 2016
To estimate the 20  Prevalence estimates with 95% CIs (error bars) were weighted.P < .001for trend was calculated using a weighted logistic regression model, which included survey cycle as a continuous variable and adjusted for age, sex, and race/ethnicity.

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Twenty-Year Trends in Diagnosed ADHD Among US Children and Adolescents

Discussion
In a nationally representative population, we estimated that the prevalence of diagnosed ADHD among US children and adolescents was 10.2% in 2016.The prevalence differed significantly by age, sex, race/ethnicity, family income, and geographic region.Similar to our findings, several previous studies also reported sex [8][9][10][11][12][13][14]19 and racial/ethnic differences 15,[24][25][26] in the prevalence of ADHD.  b values for trends were calculated using weighted logistic regression models, which included survey cycle as a continuous variable and adjusted for age, sex, and race/ ethnicity.
c P values for interaction were calculated by including multiplicative terms of each stratum variable with survey cycle in the aforementioned logistic regression models.
d The numbers of participants overall and with ADHD were unweighted.

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Twenty-Year Trends in Diagnosed ADHD Among US Children and Adolescents Over the 20-year period from 1997 to 2016, we found a significant increase in the prevalence of diagnosed ADHD from 1997-1998 to 2015-2016.We found a consistent upward trend across subgroups by age, sex, race/ethnicity, family income, and geographic regions.][10][11][12][13][14][15] Taken together, these findings indicate a continuous increase in the prevalence of diagnosed ADHD among US children and adolescents.Previous studies conducted in the United Kingdom have also observed a significant increase in ADHD prevalence, although the prevalence estimates were substantially lower than those in the United States. 2 Nonetiologic factors may partly explain the apparent increase in the prevalence of diagnosed ADHD in this study.Over the past 20 years, there have been expanded continuing medical education efforts about ADHD that enhanced physicians' sensitivity to the diagnosis of ADHD.Changes in diagnostic criteria may also contribute to the increased number of children being diagnosed with ADHD. 7In particular, changes in the Diagnostic and Statistical Manual of Mental Disorders criteria that established the predominately inattentive presentation of ADHD led to significantly increased diagnosis in girls, who often fail to demonstrate classic symptoms.In addition, increased public awareness, improved access to health services, and improved referral from primary care and communities to specialty mental health services may increase the likelihood of ADHD being identified on screening and diagnosis. 9Increased rates of diagnosed ADHD among black and Hispanic youths might reflect increased access to care and decreased stigma in those communities for receiving an ADHD diagnosis.The execution of the Affordable Care Act may also have increased access to care in lower socioeconomic status and minority groups.There is a common perception that ADHD is overdiagnosed in the United States, but this perception was not supported by scientific evidence based on review of prevalence studies and research on the diagnostic process. 27 remains to be understood how much of the observed apparent increase in diagnosed ADHD was attributed to etiologic factors; ADHD has a genetic component with an estimated heritability of 70% to 80%. 21In addition to genetic risk factors, environmental risk factors are believed to contribute to the development of ADHD. 1,283][34] Environmental contamination, such as lead, organophosphate pesticides, and polychlorinated biphenyls exposure, during prenatal and/or postnatal periods is a possible risk factor for ADHD. 30In addition, nutritional deficiencies (eg, zinc, magnesium, and polyunsaturated fatty acids) may also be implicated in the development of ADHD. 1 The contributions of these nongenetic and genetic risk factors to the etiologic source of ADHD, both separately and jointly, warrant further investigation.

Strengths and Limitations
This study has several strengths.First, this study was based on a nationally representative sample of the US population, which facilitates the generalization of the findings to a broader population.
Second, a large sample size with a multi-racial/ethnic diverse population was available, allowing us to assess disparities in ADHD prevalence according to population characteristics.Third, with a series of nationwide population-based cross-sectional surveys, we were able to evaluate the secular trends in ADHD prevalence over a period of observations as long as 20 years.
This study has some limitations.First, ADHD was ascertained by parent-reported physicians' diagnosis, which may be subject to misreporting and recall bias.Second, we did not know whether the children and adolescents with a diagnosis of ADHD still had ADHD at the time of the survey across the survey years except in NHIS 2016.Previous studies have shown that the core symptoms of ADHD tend to decline with age, but inattentive features of ADHD are more likely to persist. 1 In NHIS 2016, we found that 85% of children and adolescents with a history of ADHD diagnosis were reported as currently having ADHD, which was similar to the number reported in a previous study. 9

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Twenty-Year Trends in Diagnosed ADHD Among US Children and Adolescents
Annual sample size of the NHIS is about 35 000

Table 1 .
Prevalence of Diagnosed ADHD in US Children and Adolescents Aged 4 to 17 Years, 2015-2016 a The numbers of participants overall and with ADHD were unweighted.bPrevalence estimates were weighted.c P values for overall differences in prevalence by stratum.

Table 2 .
Trends in the Prevalence of Diagnosed ADHD in US Children and Adolescents Aged 4 to 17 Years, 1997-2016