Overdiagnosis and underdiagnosis of attention-deficit/hyperactivity disorder (ADHD) are widely debated, fueled by variations in prevalence estimates across countries, time, and broadening diagnostic criteria. We conducted a meta-analysis to: establish a benchmark pooled prevalence for ADHD; examine whether estimates have increased with publication of different editions of the Diagnostic and Statistical Manual of Mental Disorders (DSM); and explore the effect of study features on prevalence.
Medline, PsycINFO, CINAHL, Embase, and Web of Science were searched for studies with point prevalence estimates of ADHD. We included studies of children that used the diagnostic criteria from DSM-III, DSM-III-R and DSM-IV in any language. Data were extracted on sampling procedure, sample characteristics, assessors, measures, and whether full or partial criteria were met.
The 175 eligible studies included 179 ADHD prevalence estimates with an overall pooled estimate of 7.2% (95% confidence interval: 6.7 to 7.8), and no statistically significant difference between DSM editions. In multivariable analyses, prevalence estimates for ADHD were lower when using the revised third edition of the DSM compared with the fourth edition (P = .03) and when studies were conducted in Europe compared with North America (P = .04). Few studies used population sampling with random selection. Most were from single towns or regions, thus limiting generalizability.
Our review provides a benchmark prevalence estimate for ADHD. If population estimates of ADHD diagnoses exceed our estimate, then overdiagnosis may have occurred for some children. If fewer, then underdiagnosis may have occurred.
Reply to Rohde and Polanczyk
We thank Professors Rohde and Polanczyk for their comments on our paper and welcome the opportunity to discuss the points that they raise. We believe that these issues should be discussed in as wide a forum as possible.
The estimate of prevalence of ADHD from our review was 7.2%, and from their 2014 review was 5.2%. As was pointed out in their correspondence, the difference between the two estimates is because:
1) we included as many original studies reporting the prevalence of ADHD that we could find (regardless of methodological limitations) because clinicians, parents, teachers, and media do not routinely appraise peer- reviewed publications but accept and report results regardless. We do not condone this practice but argue that prevalence estimates from many research papers (robust or otherwise) provide anchors from which people form opinions about ADHD prevalence.
2) we did not include studies using the ICD criteria as our purpose was to consider change over time using only DSM criteria. As Rohde and Polanczyk stated, the more restrictive criteria used in the ICD definition will give lower estimates of prevalence.
Given the differences in the inclusion/exclusion criteria of the two reviews, the findings of the two reviews are quite consistent with each other. There is no universal definition of ADHD, although the DSM criteria are used more widely than just the US.
We apologise for not having seen their review prior to the publication of our study. It was published on January 24th and our search was conducted on January 13th.
Conflict of Interest:
The (big) pieces that were missing: methodological limitations and over-interpretation of findings
We would like to congratulate Thomas et al.(1) for their effort in attempting to provide an estimate of the worldwide prevalence of ADHD. As they stated, this estimate is hugely important given the controversy of whether ADHD is overdiagnosed or underdiagnosed.
The authors stated several times, from the abstract to the discussion, that they are providing a benchmark pooled prevalence for ADHD. Unfortunately, we believe that this is not the case for a number of reasons, as follow. 1) To provide the prevalence rate of a disorder, one should only include studies that derive the full diagnosis for that disorder. In their review, authors included studies that used questionnaires and partial diagnosis. The fact that these characteristics were not significantly associated with the ADHD prevalence rate in multivariate meta-regression analyses is interesting, but not indicate that is valid to define ADHD caseness according to DSM just based on part of the criteria. 2) Although the authors did not present the heterogeneity for the overall prevalence rate of ADHD, it should have been of a huge magnitude, based on other data reported, such as the data on the heterogeneity of low risk of bias studies. Indeed, this is true for all the previous studies, including ours. In such conditions, a pooled estimate would be hardly considered a benchmark (2). 3) More importantly, the authors did not include studies using ICD criteria. The DSM is the official classification system only in the US. To support the idea of providing a worldwide benchmark prevalence rate for a disorder using criteria that is only clinically valid in one country is at minimum surprising, unless authors invoke the very superficial argument that the nomenclature used by ICD (Hyperkinetic Disorder) is different than the one used by DSM (ADHD).
By the way, differently than proposed by the authors, we believe that the main reason for the difference in the pooled prevalence rate found in our study (5.2%) (3) in comparison to the one in their survey (7.2%) is the inclusion of studies using ICD in ours. Any clinician or investigator in the ADHD field knows that ICD criteria for ADHD are more restrictive than DSM criteria.
Finally, the authors also stated several times throughout the paper that they conducted the first meta-analysis investigating if there has been a change in the prevalence of ADHD over time (see, for example, first phrase of the Discussion). It is very surprising that authors from a Centre for Research in Evidence-Based Practice conducting a systematic review and meta-analysis that they self-denominate as the one providing the benchmark pooled prevalence for ADHD did not come across to our study addressing exactly this research question (change in the prevalence of ADHD over time) published one year earlier than the date their study was accepted (4).
References: 1. Thomas R, Sanders S, Doust J, Beller E, Glasziou P. Prevalence of Attention-Deficit/Hyperactivity Disorder: A Systematic Review and Meta- analysis. Pediatrics 2015 Mar 2. Higgins J, Thompson S, Deeks J, Altman D. Statistical heterogeneity in systematic reviews of clinical trials: a critical appraisal of guidelines and practice. J Health Serv Res Policy 2002;7(1):51-61. 3. Polanczyk G, de Lima MS, Horta BL, Biederman J, Rohde LA. The worldwide prevalence of ADHD: a systematic review and metaregression analysis. Am J Psychiatry 2007;164(6):942-8. 4. Polanczyk GV, Willcutt EG, Salum GA, Kieling C, Rohde LA. ADHD prevalence estimates across three decades: an updated systematic review and meta-regression analysis. Int J Epidemiol 2014;43(2):434-42.
Conflict of Interest:
Prof. Luis Augusto Rohde has been a member of the speakers' bureau/advisory board and/or acted as a consultant for Eli-Lilly, Janssen-Cilag, Novartis and Shire in the last three years. He receives authorship royalties from Oxford Press and ArtMed. He has also received travel awards from Shire for his participation of the 2014 APA meeting. The ADHD and Juvenile Bipolar Disorder Outpatient Programs chaired by him received unrestricted educational and research support from the following pharmaceutical companies in the last three years: Eli-Lilly, Janssen-Cilag, Novartis, and Shire. Prof. Guilherme V. Polanczyk has served as a speaker and/or consultant to Shire, developed educational material for Janssen-Cilag and Shire, receives authorship royalties from Editora Manole.