Studying the Epidemiology of Attention-Deficit Hyperactivity Disorder: Screening Method and Pilot Results

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1 ORIGINAL RESEARCH Studying the Epidemiology of Attention-Deficit Hyperactivity Disorder: Screening Method and Pilot Results An drew S Row land, PhD 1, David M Um bach, PhD 2, Karen E Ca toe, MPH 3, Lil Stallone, MPH 4, Stu art Long, BS 5, David Ra bi ner, PhD 6, A J Naf tel, MD 7, De bra Panke, DO 8, Rich ard Faulk, MD 9, Dale P San dler, PhD 10 Objective: As part of a larger epidemiologic study of risk factors for attention-deficit hyperactivity disorder (ADHD), this pilot study combined parent and teacher information to estimate ADHD prevalence among elementary school children in a North Carolina county. The methods developed for this study and the pitfalls we encountered illustrate the challenges involved in conducting population-based studies of ADHD. Methods: We employed 2-stage screening using DSM-IV criteria. Teachers completed behaviour-rating scales for all children. We then administered a structured telephone interview to parents of potential cases. We screened 362 of 424 (85%) children in grades 1 to 5 in 4 schools. Results: According to parent reports, 43 children (12%) had previously been diagnosed with ADHD by a health professional. Thirty-four children (9%) were taking ADHD medication. Forty-six children (12.7%) met study case criteria for ADHD, based on combined teacher and parent reports. Of the 46 cases, 18 (39%) had not been previously identified. Eight previously diagnosed children, however, did not meet case criteria. After we adjusted for nonresponse, the estimated prevalence was 16% (95%CI, 12% to 20%). Conclusions: These data suggest that the DSM-IV prevalence of ADHD has been substantially underestimated, although the true prevalence in this population may be less than the 16% estimated here. Population-based studies of ADHD are feasible and may provide important information about practice and treatment patterns in community settings, as well as a broader understanding of the etiology and life course of this common disorder. (Can J Psy chia try 2001:46: ) Key Words: attention-deficit hyperactivity disorder, methods, epidemiology, stimulant medication, prevalence, psychiatric disorders, children Over their life time, chil dren with attention- deficit hy per - ac tiv ity disorder (ADHD) are at in creased risk for aca - demic fail ure, be hav ioural prob lems, sub stance abuse, ac ci dents, di vorce, and mental dis or ders (1 5). There is also con cern, how ever, about the wide spread use of stimu lant medi ca tion to treat chil dren di ag nosed with ADHD (6). De - Manuscript received June 2001 and accepted October Senior Staff Fellow, Epidemiology Branch, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, North Carolina. 2 Statistician, Biostatistics Branch, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, North Carolina. 3 Study Manager, CODA/Westat Inc, Durham, North Carolina. 4 Study Manager, CODA/Westat Inc, Durham, North Carolina. 5 Programmer, CODA/Westat Inc, Durham, North Carolina. 6 Senior Research Scientist, Center for Child and Family Policy, Duke University, Durham, North Carolina. 7 Director, Division of Child and Adolescent Psychiatry, Department of Psychiatry, University of North Carolina School of Medicine, Chapel Hill, North Caro - lina. 8 Child Psychiatry Resident, Department of Psychiatry, University of North Carolina School of Medicine, Chapel Hill, North Carolina. 9 Child Psychiatry Resident, Department of Psychiatry, University of North Carolina School of Medicine, Chapel Hill, North Carolina. 10 Acting Branch Chief, Epidemiology Branch, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, North Carolina. Address for correspondence: Dr AS Rowland, MPH Program, Dept of Family and Community Medicine, University of New Mex ico Health Sciences Cen ter, 2400 Tucker NE, Albuquerque, NM arowland@salud.unm.edu Can J Psychiatry, Vol 46, December

2 932 The Canadian Journal of Psychiatry Vol 46, No 10 spite the enor mous medi cal and emo tional costs of ADHD and the con tro ver sies sur round ing its treat ment, knowl edge about preva lence and risk fac tors is lim ited (7). For ex am ple, es ti mates of the sex ra tio for ADHD vary widely (8), and few stud ies have ad dressed whether prevalence varies by eth nic - ity or so cial class. In part, these knowl edge gaps re flect the dif fi culty in volved in es tab lish ing a re li able case defi ni tion and screening meth od ol ogy for epi de mi ol ogic studies. The es ti mated preva lence of ADHD among school-age chil - dren in the US is be tween 3% and 5%,ac cord ing to the DSM- IV (9). The Na tional In sti tutes of Health Con sen sus Confer - ence on ADHD and sev eral re cent re views pro vide simi lar es - ti mates (7,9,10). Yet, de spite be ing fre quently quoted, the 3% to 5% preva lence es ti mate is poorly docu mented: ADHD preva lence rates vary widely, de pend ing on the population as sessed and the meth od ol ogy used (11,12). We are con duct ing a population- based study that in volves screen ing all ele men tary chil dren in John ston County, North Caro lina, to es ti mate ADHD preva lence and identify pos si ble risk fac tors. To do this, we en gaged the DSM- IV di ag nos tic cri te ria for ADHD, adapt ing them for use in a large- scale field study. Here, we de scribe a method for con duct ing epi de mi ol - ogic stud ies of ADHD that com bines in for ma tion from par - ents and teach ers and clas si fies chil dren be ing treated with stimu lant medi ca tion based on their be hav iour dur ing the year be fore they be gan treat ment. We use data from a large pi - lot study to pro vide a pre limi nary es ti mate of ADHD preva - lence among ele men tary school chil dren in this semi ru ral county, to de scribe prac tice and treat ment pat terns in a com - mu nity set ting, and to il lus trate the meth odo logi cal chal - lenges in volved in con duct ing this type of re search. Methods Popu la tion John ston County, North Caro lina, is eco nomi cally di verse: some re gions are ru ral and poor, and oth ers are sub ur ban and mid dle class. With guidance from the school ad mini stra tion, we chose for our pi lot study 4 ele men tary schools that re flect this di ver sity. All chil dren in these schools were eli gi ble, ex - cept those in self- contained class rooms for chil dren with autism, IQs be low 70, or se vere de vel op men tal dis abili ties. Prin ci pals in each school des ig nated 1 teacher in each grade 1 to 5 to par tici pate. One school did not have a fifth grade, and 1 fourth- grade teacher did not par tici pate; there fore, 424 stu - dents from 18 class rooms were eli gi ble. Re cruit ment The pi lot study was con ducted dur ing the summer and fall of A let ter and con sent form were sent to the par ents of each child, in vit ing them to join the study. We in cluded 2 ques tions: 1) Has a doc tor or psy cholo gist ever told you that your child has attention- deficit dis or der, attention- deficit hy - per ac tiv ity dis or der or hy per ac tiv ity? and 2) Is your child cur rently tak ing any medi cine pre scribed by a doctor to help with symp toms of ADHD (for ex am ple, in at ten tion or hy per - ac tiv ity) or to help the child s mood or be hav iour? Chil dren were cate go rized as taking ADHD medi ca tion if their par ents an swered yes to both ques tions. For ex am ple, a child taking an an ti de pres sant who pre vi ously had been di ag nosed with ADHD was clas si fied as tak ing ADHD medi ca tion. Con - versely, a child tak ing an an ti de pres sant who had not been pre vi ously di ag nosed with ADHD was not so classified. As sess ment of ADHD Over all Ap proach. The over all strat egy for iden ti fy ing cases in volved 2 phases. In Phase 1, po ten tial cases were iden ti fied ei ther from teacher be hav iour rat ings or from pa ren tal re port of a child s cur rent use of medi ca tion to treat ADHD. In Phase 2, par ents or guardi ans of po ten tial cases were in ter viewed by tele phone. We de ter mine fi nal case status using data from both phases. Ta ble 1 sum ma rizes the DSM- IV cri te ria for ADHD and how we adapted those cri te ria for our study. This pro cess is de scribed in detail be low. Teacher Screen ing. Home room teach ers com pleted a symp - tom and im pair ment check list (Ap pen dix 1) for each child whose par ents con sented. Teach ers com pleted the screening in stru ment in June and an swered items that best de scribe the child s be hav iour since the be gin ning of the year. Items were worded al most iden ti cally to 2 simi lar, widely used DSM- IV teacher check lists with good psy cho met ric prop er ties (13,14). How ever, we changed the re sponse cate go ries to fit DSM- IV cri te ria more closely: these count symp toms as pres - ent when they oc cur of ten (8). Most ex ist ing rating in stru - ments use a 4- point scale with of ten and very often as the 2 top cate go ries. Some re search ers have counted both of ten and very of ten as posi tive symptom re ports whereas oth ers have counted only very of ten a choice that can sig nifi - cantly shift symptom prevalence rates (15). Our screening form had a 4- point re sponse scale: never, hardly ever, some of the time, and a lot of the time. Only a lot of the time was con sid ered a posi tive re sponse. We thought this re - sponse scale would pro vide a more con ser va tive preva lence es ti mate than scales that counted of ten and very of ten as evi dence of posi tive symp toms. Be cause the wording of the ques tions we used was very simi lar to those of es tab lished in - stru ments (13,14), a full psy cho met ric analy sis of our in stru - ment is de ferred until the en tire study is com pleted. In a pre limi nary cor re la tional analy sis, how ever, the Cron bach s al pha co ef fi cients for the in at ten tive and hyperactiveimpulsive subscales were 0.96 and 0.95, re spec tively, in di - cat ing strong in ter nal con sis tency among items on these subscales. Chil dren were con sid ered im paired at school if their teacher rated them be low av er age in any of 7 areas: read ing, mathe - mat ics, written ex pres sion, or gan iza tional skills, as sign ment com ple tion, peer re la tions, or fol low ing rules. Chil dren were

3 December 2001 Studying the Epidemiology of ADHD: Screening Method and Pilot Results 933 Table 1. Comparison of DSM-IV criteria for attention-deficit hyperactivity disorder (ADHD) and our operational definition DSM-IV criteria A. 6 unique symptoms of inattention or of hyperactivity impulsivity Operational definition A1. At least 3 inattentive symptoms or 3 hyperactive-impulsive symptoms were reported by the child s teacher and by the parent. In addition, when the teacher and parent reports were combined, there were at least 6 unique symptoms of inattention or 6 unique symptoms of hyperactivity-impulsivity. or: A2. Children taking stimulant medicine to treat ADHD were considered cases unless a parent reported that the child showed < 6 symptoms and no impairment during the year before starting medicine. B. Evidence of impairment in 2 settings and clinically significant impairment in social or academic functioning B1. Evidence existed of impairment at school either in academic performance or behaviour and evidence existed of impairment related to symptoms at home as reported from ADHD questions from parent Diagnostic Interview Schedule for Children (DISC). and: B2. Evidence of severe impairment in at least 1 area of social or academic functioning was reported by either the teacher or the parent. C. Symptoms present for at least 6 months C1. Symptoms were reported present for most of the school year by the teacher who filled out the questionnaire in June. and: C2. Symptoms were reported present for at least 6 months according to DISC questions. D. Symptoms began before age 7 years D. Age criterion was not used, based on experience of DSM-IV field trials (but data were presented for age-of-onset). E. Symptoms not explained by other psychopathology F. Subtypes assigned by number of symptoms of inattention or hyperactivity impulsivity and impairment E. Children from self-contained classrooms for severe developmental disabilities or with IQ below 70 were excluded. Child Behaviour Checklist was used as a screen for other comorbidity. F. Subtypes were assigned using DSM-IV criteria, except that children taking medicine to treat ADHD were assigned subtypes based on parent report of the child s symptoms and impairment in the year before beginning medication. de fined as se verely im paired if they were rated far be low av - er age in any area. We clas si fied chil dren as po ten tial cases if they showed 3 or more symp toms of in at ten tion or hyperactivity- impulsivity on the teacher screen as well as evi dence of im pair ment. This ap proach mir rors the scoring al go rithms used in the Di ag nos - tic In ter view Sched ule for Chil dren (DISC) Ver sion IV (16), where im pair ment ques tions are omit ted for chil dren with fewer than 3 symp toms, thus elimi nat ing any chance that they could be clas si fied as a case. Be cause ADHD medi ca tion ame lio rates symp toms, chil dren being treated with medi ca - tion were in cluded as po ten tial cases, re gard less of the number of symp toms re ported by their teach ers. Chil dren who had been pre vi ously di ag nosed with ADHD but who were not taking ADHD medi ca tion were treated dif fer ently: they were con sid ered po ten tial cases only if they met teacherre ported symp tom and im pair ment cri te ria. Par ent Screen ing. Par ents of po ten tial cases were asked to par tici pate in a tele phone in ter view that in cluded the ADHD mod ule adapted from the DISC-IV (16). We re tained the DISC ques tions ver ba tim but omit ted ques tions about par ent per cep tion of the child s symp toms at school (be cause symp - tom in for ma tion was col lected di rectly from the teacher) and those few ques tions not in cluded in the DISC scor ing al go rithm. We as sessed im pair ment at home using 5 ques tions from the DISC- IV. The DISC- IV probes how of ten the care giver be - came upset with the child and how of ten the child had prob - lems do ing things with the fam ily, doing things with other chil dren, doing home work or grades, or felt bad or up set. Fol - low ing the DISC al go rithm, we re garded the re sponses some of the time, a lot of the time, bad, or very bad as evi - dence of im pair ment. The re sponses a lot of the time or very bad were clas si fied as se vere im pair ment. We asked par ents of chil dren taking ADHD medi ca tion about their child s symp toms and im pair ment dur ing the year be - fore treat ment be gan. These chil dren were con sid ered cases only if they had dis played 6 or more symp toms of in at ten tion or hy per ac tiv ity to gether with im pair ment from those symp - toms during the year be fore treat ment be gan. Com bin ing Teacher and Par ent Re ports. To be de clared a case, a child had to dis play at least 6 of the 9 in di vid ual DSM- IV in at ten tive symp toms, or 6 of the 9 in di vid ual DSM- IV hyperactive- impulsive symp toms on com bined parent and teacher re port (Ta ble 1). These symp toms had to have been pres ent for 6 months or longer. In ad di tion, we re quired chil - dren to show at least 3 symp toms of in at ten tion or 3 symp - toms of hyperactivity- impulsivity, to gether with im pair ment from those symp toms, in each set ting. This ad di tional con - straint required a modest level of con cor dance be tween par - ent and teacher re ports re gard ing symp toms and im pair ment. Even in clini cal prac tice, di ag no sis is dif fi cult when dif fer ent re spon dents give widely dis crep ant re ports. For epi de mi ol - ogic pur poses, re quir ing mod er ate agreement be tween par - ents and teach ers seemed ap pro pri ate; er rors would be to ward un deras cer tain ment and se lect ing more clearly de fined, se - vere cases. The DSM- IV re quires not only im pair ment that is pres ent in 2 set tings but also clear evi dence of clini cally sig - nifi cant im pair ment in so cial, aca demic, or oc cu pa tional

4 934 The Canadian Journal of Psychiatry Vol 46, No 10 Table 2. Prevalence of diagnosed ADHD by ethnicity and sex, based on parent report prior to screening n Pre vi ously di ag - nosed with ADHD Tak ing medi - cine to treat ADHD n (%) n (%) Whole sample 359 a 43 (12.0) 34 (9.5) Ethnicity White (12.2) 28 (9.8) African American 54 7 (13.0) 5 (9.3) Hispanic 16 1 (6.3) 1 (6.3) Other 2 0 (0.0) 0 (0.0) Sex Boys (16.0) 25 (13.8) Girls (7.9) 9 (5.1) a In for ma tion on prior di ag no sis not pro vided by par ents of 3 chil dren Phase 1: Flow Dia gram of Screen ing Process 424 eli gi ble chil dren. 362 chil dren (85%) screened by teacher. 86 po ten tial cases iden ti fied. Phase 2: 34 by medi ca tion and 52 by symp toms and im pair ment. (21 of the 34 chil dren on medi ca tion also met po ten tial case cri te ria by symp toms and im pair ment). Par ents of 67 (78%) po ten tial cases in ter viewed 28 of those on medi ca tion and 39 iden ti fied by symp toms. 46 ADHD cases iden ti fied after both screen ing phases 21 from the symp toms group and 25 from the medi ca tion group. Figure 1. Phase 1 included parent-reported history of di ag no sis ADHD from a health professional and current medication treatment for ADHD as well a teacher-completed behav - iour check list on each student. Phase 2 involved a struc - tured interview with parent or guardian that included the ADHD module adapted from the Diagnostic Interview Schedule for Children, (DISC), Version IV, developed by the National Institutes of Mental Health. func tion ing. We in ter preted this re quire ment to mean that the child was im paired at school and at home and se verely im - paired in at least 1 set ting. DSM- IV cri te ria for ADHD specify that some symp toms which caused im pair ment must have been pres ent be fore age 7 years. We used the DISC -IV ques tions to es tab lish age- ofonset of symp toms. In the DSM- IV field tri als many chil - dren par ticu larly those with in at ten tive symp toms who oth er wise met case cri te ria did not meet the age- of- onset cri - te rion (17). In re sponse, sev eral lead ing ADHD ex perts sug - gested re vis ing the cri te rion (17,18). Con se quently, we did not rule out cases based on age- at- onset but in stead re port re - sults strati fied by age- at- onset. Results Of 424 par ents, 362 (85%) con sented to have their child s teacher com plete the symp tom and im pair ment check list. The sam ple was about equally di vided be tween boys and girls and was about 80% White, 15% Af ri can Ameri can, and 5% His - panic. Of the 362 par ents, 359 an swered the ques tions about their child s di ag nos tic his tory of ADHD and cur rent treat - ment. Ac cord ing to pa ren tal re port, 43 chil dren (12% of 359) had been pre vi ously di ag nosed with ADHD by a psy cholo gist or a phy si cian. Of these 43, 34 chil dren (9.5% of all chil dren in the sample) were cur rently taking ADHD medi ca tion. Of the 34 chil dren taking ADHD medi ca tion, 32 (94%) were tak - ing stimu lants (meth ylpheni date, n = 28; am pheta mine, n = 4). We found little dif fer ence in the pro por tion of African- American and White students who had been pre vi ously diag - nosed (13% vs 12%, (χ 2 = 0.03, df 1, P = 0.87) or those cur - rently taking ADHD medi ca tion (9.3% vs 9.8%; χ 2 = 0.01, df 1, P = 0.91) (Ta ble 2). Among chil dren pre vi ously di ag nosed, the boy girl ra tio was about 2:1 (χ 2 = 5.78, df 1, P = 0.02), and among chil dren cur rently tak ing medi ca tion, the boy girl ra - tio was 2.7:1 (χ 2 = 8.33, df 1, P = 0.004) (Ta ble 2). Af ter Phase 1, 86 chil dren were iden ti fied as po ten tial cases 52 based on symp toms and im pair ment alone and 34 be cause they were tak ing medi ca tion for ADHD at the time of screen ing (Fig ure 1). Par ents of 67 po ten tial cases (78%) com pleted the tele phone in ter view; this number in cluded par - ents of 82% of the chil dren on medi ca tion and 75% of those iden ti fied by symp toms alone. Af ter both phases of screen - ing, 46 of 362 chil dren in the sam ple (12.7%) met study cri te - ria for ADHD. This es ti mate does not ac count for pos si ble cases among the 19 chil dren whose par ents did not com plete the tele phone in ter view. In ef fect, it as sumes that no ad di - tional cases would be found among those 19 chil dren. We did not want to make this as sump tion, or the other ex treme as - sump tion that all 19 would have be come cases, so we took a mid dle ground. We ad justed the preva lence data by assuming that the con ver sion rate of po ten tial cases to ac tual cases was

5 December 2001 Studying the Epidemiology of ADHD: Screening Method and Pilot Results 935 Table 3. ADHD subtype among 46 identified cases by diagnostic history Predominately inattentive subtype Predominately hyperactive subtype All iden ti fied Pre vi ously di ag nosed Tak ing medi ca tion On set of symp toms after age 6 years n n (%) n (%) n (%) 12 4 (33.3) 3 (25.0) 6 (50.0) 1 1 (100.0) 1 (100.0) 0 (0.0) Combined subtype (69.6) 21 (63.6) 4 (12.1) To tal (60.9) 25 (54.4) 10 (21.7) be fore treat ment be gan, and the third child showed many symp toms but was not se verely impaired. Of the 46 cases, 12 (26%) had the pre domi nately in at ten tive sub type of ADHD, 1 (2.0%) had the pre domi nately hy per ac - tive sub type, and 33 (72%) had the com bined sub type Ta ble 3). Chil dren with the pre domi nately in at ten tive sub type were about one- half as likely as chil dren with the com bined sub - type to have been pre vi ously di ag nosed (Fish er s ex act P = 0.04) and about 40% as likely to be taking medi ca tion (Fish - er s ex act P = 0.04). Chil dren with the pre domi nately in at ten - tive sub type ap peared more likely than chil dren with the com bined sub type to be aca demi cally im paired, but less likely to be be hav iour ally im paired (Fig ure 2) Inattentive Subtype (N=12) Combined Subtype (N=33) For 10 (22%) of the 46 cases, symp tom onset was at age 7 years or older (Ta ble 3). There fore, these cases did not meet strict DSM- IV cri te ria for ADHD. The me dian age of on set was 4 years for both the com bined sub type and the hyperactive- impulsive sub type but 6.5 years for the in at ten - tive sub type. Com pared with the chil dren who had the com - bined sub type, chil dren with the in at ten tive sub type were about 4 times as likely not to meet the DSM- IV age- at- onset cri te ria (Fish er s ex act P = 0.01). 0 Figure 2. Teacher-rated impairment among cases by ADHD sub - type. 45/46 cases are included. The 1 case classified as predominately hyperactive-impulsive subtype was ex - cluded. Impairment was defined as below average functioning at school in any of 7 domains assessed. Only the differences in the Math (Fisher s exact P = 0.005) and the Peer Relations (Fish er s exact P = 0.02) domains achieve statistical significance (all other domains have P > 0.15), but the power of all tests is limited by the small number of cases in this pi lot study the same among the chil dren of par ents who were in ter viewed and those who were not in ter viewed. Be cause this pro por tion dif fered be tween chil dren tak ing ADHD medi ca tion and chil - dren iden ti fied by symp toms and im pair ment alone, we ac - counted for that dif fer ence in our ad just ment; we ultimately es ti mated that ap proxi mately 12 ad di tional cases would have been found had all par ents of po ten tial cases par tici pated. The re sult ing ad justed prevalence was 16.1% (95% boot strap CI, 12% to 20%). Eight een of the 46 cases (39%) we iden ti fied had never been di ag nosed with ADHD. Eight of the 43 chil dren (19%) who pre vi ously had been pro fes sion ally di ag nosed did not meet the study case cri te ria. Three chil dren tak ing ADHD medi ca - tion did not be come cases. Ac cord ing to their par ents, 2 of these chil dren showed few symp toms at home dur ing the year By defi ni tion, cases showed some im pair ment at school and at home (or, for those taking medi ca tion, at least im pair ment at home). Most cases were im paired across mul ti ple do mains. Teach ers rated 74% of the cases, but only 13% of the non - cases, as im paired in at least most (4 or more) of the 7 impair - ment ar eas probed (Fig ure 3). When we first gath ered con sent, 34 chil dren were taking ADHD medi ca tion. De spite medi ca tion, 13 chil dren (38%) showed at least 6 symp toms of in at ten tion or hyperactivityimpulsivity with im pair ment at school. Discussion Our es ti mate of the ADHD preva lence in this population (16%) is over 3 times the preva lence cited in the DSM- IV (8). Over 12% of the chil dren sam pled had been di ag nosed with ADHD by a phy si cian or psy cholo gist, and over 9% were tak - ing medi ca tion for ADHD be fore we be gan screen ing. What ac counts for the dis crep ancy be tween the preva lence we re - port and the most com mon es ti mate? One rea son for the dis crep ancy is that DSM- IV cri te ria are more in clu sive than those used in DSM- III-R. Two stud ies that rated the same chil dren us ing cri te ria from DSM- III-R and DSM- IV re ported that simply us ing the new, more in clu - sive cri te ria in creased preva lence by about 60% (19,20). This find ing raises the ques tion whether these new cri te ria are now too in clu sive. Nev er the less, the im pair ment data we col lected sug gest that, as a group, case chil dren en coun tered sub stan tial prob lems in their lives across mul ti ple do mains.

6 936 The Canadian Journal of Psychiatry Vol 46, No Figure 3: Number of Teacher-rated Impairment Domains by Case Status 60.1 no impairment 1-3 impairment domains 4-7 impairment domains 73.9 same de vel op men tal level and thus have a ref er ence frame - work for spot ting de fi cien cies in at ten tion or be hav iours that are not age- appropriate. Par ent and teacher re ports are im por - tant com po nents of the clini cal di ag nos tic workup of ADHD (1,9,27). Us ing both par ent and teacher re ports may be opti - mal when screen ing for ex ter nal iz ing be hav iours like ADHD (28,29). Nev er the less, only a few of the ADHD preva lence stud ies have com bined teacher and par ent re ports (30 33) Non-Cases (N=301) Other studies have re ported ADHD medi ca tion rates in the range re ported here. A study of 2 Vir ginia school sys tems re - ported that 8% to 10% of all sec ond- to fifth- grade chil dren were re ceiv ing ADHD medi ca tion at school (21). This study did not in clude chil dren who only took their medi ca tion at home, so the authors con cluded that their es ti mates of the preva lence of medi ca tion treat ment were proba bly con ser va tive. Few stud ies have used DSM- IV cri te ria to screen chil dren sys tem ati cally through the schools. Es ti mates from stud ies that have used teacher symp tom reports sug gest that overall ADHD preva lence among US ele men tary school chil dren ranges from 8% to 16% (15,19,22). An im por tant limitation of most school- based stud ies is that they limit ob ser va tions to a child s be hav iour in only 1 set ting. ADHD preva lence esti - mates are par ticu larly sen si tive to meth odo logi cal nuances, in clud ing who is being asked to re port symp toms (23,24) and how dis crep ant in for ma tion from par ents, teach ers, or chil - dren is com bined (25). Be cause par ent and teacher re ports of - ten dis agree, the DSM-IV re quires evi dence of symp toms and im pair ment in 2 set tings. Using mul ti ple in for mants has be - come an es sen tial epi de mi ol ogic tool for as sess ing childhood psy cho pa thol ogy (26). Teach ers are par ticu larly valu able in - for mants be cause they daily ob serve many chil dren at the Cases (N=46) Figure children were in the sample, including 46 cases and 301 noncases: 15 children who had been diagnosed with ADHD but did not complete a parent interview were ex - cluded. The 7 domains were read ing, math, written ex - pression, assignment completion, organizational skills, peer relations, and following directions or rules. A small group of cases (6.5%) did not show impairment at school; these were children who were taking ADHD medication. This group must have showed impairment at home during the year before starting medication to be considered cases. The difference in distribution between non-cases and cases illustrated is statistically significant (χ 2 = 94.8, df 2; P < ) An other limi ta tion of many school- based ADHD preva lence stud ies is that they do not ex plic itly ad dress how to cate go rize chil dren taking ADHD medi ca tion. In most, chil dren taking medi ca tion who do not ex hibit 6 or more symp toms are im - plic itly counted as non cases (15,19,20,22,32,33). Our study is un usual in in clud ing these chil dren as po ten tial cases un til ei ther con firmed or ruled out by pa ren tal screen (showing symp toms and im pair ment in the year be fore start ing treat - ment). Our study pro ce dures were con ser va tive in re quir ing both teach ers and par ents to re port at least 3 symp toms of in at ten - tion or hyperactivity- impulsivity and im pair ment. Deciding the best way to re solve con flict ing re ports from among in for - mants re mains one of the key is sues in as sess ing childhood psy cho pa thol ogy (26,29). DSM- IV cri te ria are vague on this is sue: a few re search ers have in ter preted the DSM- IV cri te ria to mean that 6 symp toms must be present in each of 2 set tings (32), but in the DSM- IV field tri als symp toms re ported by dif - fer ent re spon dents were sim ply com bined (34). We sus pect the latter method more closely ap proxi mates how the cri te ria are usu ally in ter preted in clini cal prac tice. Nev er the less, other re search ers have sug gested more strin gent cri te ria for com bin ing symp toms across do mains, such as re quir ing at least 4 symp toms in one do main and 6 in the other (35). Fu ture epi de mi ol ogic studies of ADHD would bene fit from ad di - tional em piri cal and theo reti cal re search on the best way to com bine symp tom reports from par ents and teach ers. Our ap proach for es ti mat ing prevalence is the first to use a DSM- IV based in stru ment in each screen ing phase, to in cor - po rate both teacher and par ent re ports, and to ad dress the dif - fi cult prob lem of how to clas sify chil dren re ceiv ing medi ca tion for ADHD. Are the Stan dard Preva lence Es ti mates Ar ti fi cially Low? The stan dard es ti mates of ADHD preva lence may be ar ti fi - cially low. Some of the most popu lar rating scales to as sess at - ten tion and be hav iour prob lems in chil dren for ex am ple the Child Be hav iour Check list (36) or the Con ners Rat ings Scales (37) rely pri mar ily on a di men sional ap proach to iden tify cases. These in stru ments re lia bly iden tify chil dren with the high est number of symp toms but should not be used to es ti mate preva lence because, by defi ni tion, they de fine cases as the up per tail of a dis tri bu tion (that is, the top 2% to 5%). As oth ers have writ ten, if the DSM- IV cri te ria rep re - sent the cur rent, con ven tional stan dard by which a di ag no sis

7 December 2001 Studying the Epidemiology of ADHD: Screening Method and Pilot Results 937 of ADHD is made, it is pref er able to use this ap proach rather than in fer ring, for ex am ple, that a child who scores 2 stan - dard de via tions above the mean on the hy per ac tiv ity in dex of a (non- DSM- based) scale will also dem on strate symp toms di ag nos tic of ADHD (14). Many re search ers have cho sen a di men sional rather a cate gori cal ap proach to con cep tu al ize ADHD and other forms of psy cho pa thol ogy. De spite the value of this con cep tual frame work (38), we sug gest that there is an im por tant role for categorical- based epi de mi ol ogic stud ies that en gage DSM cri te ria. For ex am ple, it is im por tant to use in de pend ent di ag nos tic cri te ria to de fine ADHD to avoid in tro duc ing cir cu lar ity into preva lence es ti mates. DSM- IV is vague about the defi ni tion of im pair ment and clini cally sig nifi cant im pair ment. How these terms are de - fined can have a major im pact on prevalence es ti mates. For ex am ple, one study using DSM- IV symptom cri te ria re ported an ADHD preva lence of 16.1% in a school popu la tion. When the authors then used a di men sional ap proach to de fine clini - cal im pair ment, how ever, the ADHD preva lence dropped to 6.8% (15). In our pi lot study, we used an im pair ment scale that asked teach ers whether learning or in ter per sonal prob - lems were be low av er age or far be low av er age. In the full study, we de cided to ask whether the be hav iours caused a prob lem and, if so, how big a prob lem; we felt this might bet - ter capture the con cept of im pair ment. Ad di tional data are needed to guide fu ture ef forts to re fine these con cepts. In the in terim, it would be helpful to de velop a re search con sen sus about how to as sess im pair ment in epi de mi ol ogic studies. Limi ta tions We con ducted this study in 2 phases. In the first phase, 85% of the eligible chil dren were screened by their teach ers. We do not have good data on who the non re spon dents were and whether they dif fered from chil dren whose par ents con sented to the screen ing. Re sponses from some of the non par tici pat - ing par ents in di cated that they had chil dren with many ADHD symp toms but that they did not want them iden ti fied and pos si bly stig ma tized. Other non par tici pat ing par ents told us they did not think the study was rele vant be cause their chil - dren had few symp toms. On bal ance, we do not think the non - re spond ers were dif fer ent from the re spond ers, but we can not rule out some form of se lec tion bias. In the sec ond phase of the study, par ents were ad min is tered a tele phone in ter view; we were un able to in ter view 22% of them, either be cause we could not reach them or be cause they re fused to par tici pate. At this stage, the situa tion dif fered from Phase 1 be cause we had teacher be hav iour rat ings and par ent data about whether the child had pre vi ously been di ag nosed or was tak ing medi - ca tion to treat ADHD. Al though we cannot be sure about the im pact of the non re spond ing par ents, we were able to use the in for ma tion we had al ready col lected to ad just our preva lence es ti mates. Be cause this study was lim ited to a sin gle county, our re sults might not be gen er aliz able. Yet, we have no rea son to be lieve that John ston County has an un usu ally high rate of ADHD, com pared with the rest of North Caro lina. In fact, treat ment rates may be higher in ur ban ar eas or in coun ties with more medi cal and mental health care pro vid ers. The de mog ra phy of John ston County is simi lar to that of North Caro lina as a whole in terms of age, in come, and eth nic com po si tion. For ex am ple, in 1990, about 25% of the John ston County popu la - tion was un der age 18 years vs 24% in North Caro lina as a whole; 32% vs 27% had in comes less than $15 000; and 18% vs 22% were Af ri can Ameri can (39). We con sid ered whether a few lo cal cli ni cians might be see ing most of the chil dren and dis pro por tion ately in flu enc ing preva lence es ti mates. We iden ti fied over 30 sepa rate prac ti tio ners in volved in evalu at - ing or treat ing these chil dren, how ever, and none di ag nosed more than 2 chil dren. Be cause the teach ers who par tici pated in the pi lot study were vol un teers and not ran domly se lected, our preva lence esti - mate may be in flated. The teach ers who vol un teered may have been more likely to have chil dren with ADHD as signed to their classes, or they may have over re ported symp toms: pre limi nary data from the first year of the larger study sug gest that the teach ers in cluded in the pi lot study were more likely to have more chil dren pre vi ously di ag nosed with ADHD in their class room. In the larger study, the preva lence of pre vi - ously di ag nosed and treated ADHD ap pears to be lower than that ob served in the pi lot, but still sub stan tially higher than the often- cited 3% to 5% es ti mate. An other im por tant fea ture of our preva lence es ti mate is that 22% of the cases de vel oped symp toms and im pair ment af ter age 6 years, yet were still in - cluded. If our data are com pared with other DSM- IV based preva lence es ti mates, this fea ture must be ac counted for. There fore, we sug gest that the meth ods and ap proach of this pi lot study may be a more im por tant con tri bu tion than the preva lence es ti mate. Our meth ods for es ti mat ing preva lence did not at tempt to rule out un der ly ing con di tions that might mimic ADHD, with a sin gle ex cep tion: we ex cluded chil dren placed in selfcontained class rooms for those with se vere de vel op men tal dis abili ties. What we have pre sented are screening es ti mates, not true di ag no ses, be cause the chil dren were not seen by a cli ni cian who could rule out other un der ly ing dis or ders. We as sume that many of our cases had comorbid con di tions such as de pres sion, anxi ety, or learning dis or ders (10,11). We do not know what pro por tion of our iden ti fied cases were false posi tives (that is, chil dren with un der ly ing con di tions, such as ab sence sei zures or adverse re sponse to medi ca tion, that mim icked ADHD symp toms). Al though these con di tions proba bly did not ac count for a large pro por tion of our cases, we are plan ning a vali da tion com po nent to the larger study to ad dress this question. Clini cal and Pub lic Health Im pli ca tions In our sam ple, 19% of the chil dren pre vi ously di ag nosed with ADHD did not meet our study case criteria. This ob ser va tion

8 938 The Canadian Journal of Psychiatry Vol 46, No 10 Today s date: / / Child s Sex: 1 = Male 2 = Female Child s Race (circle): 1 = White 2 = Black Appendix 1. Attention and behaviour screener to be completed by child s teacher 3 = American Indian/Alaskan Native 4 = Asian or Pacific Islander 5 = Other, specify 6 = Don t know Is he/she also Hispanic? 1 = Yes 2 = No How long have you had this child in your class? How many hours per day do you teach this child? Please circle the number that best describes this child s behaviour since the beginning of the school year. Never Hardly ever Some of the time A lot of the time B1. Fails to give close attention to details or makes careless mistakes in schoolwork B2. Has difficulty sustaining attention in tasks or play activities B3. Does not seem to listen when spoken to directly B4. Does not follow through on instructions and fails to finish schoolwork B5. Has difficulty organizing tasks and activities B6. Avoids tasks (for example, schoolwork, homework) that require mental effort B7. Loses things necessary for tasks or activities B8. Is easily distracted B9. Is forgetful in daily activities B10. Fidgets with hands or feet or squirms in seat B11. Leaves seat in classroom or in other situations in which remaining seated is expected B12. Runs about or climbs excessively in situations in which it is inappropriate B13. Has difficulty playing or engaging in leisure activities quietly B14. Is on the go or acts as if driven by a motor B15. Talks excessively B16. Blurts out answers before questions have been completed B17. Has difficulty awaiting turn B18. Interrupts or intrudes upon others B19. Loses temper B20. Argues with adults B21. Actively defies or refuses to comply with adults requests or rules B22. Deliberately does things for his or her mistakes or behaviour B23. Blames others for his or her mistakes or behaviour B24. Touchy or easily annoyed by others B25. Angry or resentful B26. Spiteful or vindictive B27. Bullies, threatens, or intimidates others B28. Initiates physical fights B29. Truant from school B30. Lies or breaks promises to obtain goods or favours or to avoid obligations B31. Physically cruel to people B32. Others like to play with him or her Be low Average Average Above average C1. Overall academic performance C1a. reading C1b. mathematics C1c. writen expression C2. Overall classroom behaviour C2a. relationships with peers C2b. following directions or rules C2c. assignment completion C2d. organizational skills

9 December 2001 Studying the Epidemiology of ADHD: Screening Method and Pilot Results 939 is con sis tent with re search about cur rent di ag no sis and pre - scrib ing prac tices among pri mary care pro vid ers. The Pe di at - ric Re search in Of fice Set tings (PROS) net work re cently re ported that, among a sample of over 400 pe di at ric and fam - ily prac ti tio ners and al most 4000 chil dren di ag nosed with at - ten tion or hy per ac tiv ity prob lems, DSM cri te ria were used as part of the di ag nos tic pro cess only 38% of the time (40). The PROS study and our data sug gest that some chil dren may be re ceiv ing treat ment for ADHD un nec es sar ily. Al though con cern that ADHD is over di ag nosed is wide - spread (10), less has been written about pos si ble un der di ag - no sis. Sys tem atic screen ing for ADHD through school sys tems may identify chil dren who are not cur rently being treated but who might bene fit from it. In our sam ple, 39% of the cases had not been pre vi ously di ag nosed. Pub lic school sys tems may, how ever, face fi nan cial pressure not to identify ad di tional chil dren who re quire costly spe cial edu ca tion re - sources. For ex am ple, North Caro lina law pro vides spe cial edu ca tion funds for only 12.5% of the stu dents in any school dis trict; this limit is a strong dis in cen tive for iden ti fy ing ad di - tional chil dren for these pro grams (41). Moreo ver, many par - ents are re luc tant to have a child evalu ated for ADHD be cause they are con cerned that the child will be la belled or treated with stimu lant medication. Ten of 46 chil dren iden ti fied as cases had their first symp toms at age 7 ye aers or later. This was par ticu larly true for chil dren with the in at ten tive sub type. At ten tion prob lems of ten be - come ap par ent only when school work be comes more dif fi - cult (42). Un less the ADHD age- at- onset cri te rion is changed, many chil dren with ADHD symp toms and im pair ment may have dif fi culty quali fy ing for a di ag no sis and re ceiv ing the ad di tional help they may need. Some chil dren taking ADHD medi ca tion still ap pear to be symp to matic ac cord ing to teacher re port. In our sam ple, more than one- third of the chil dren be ing treated with ADHD medi ca tion showed 6 or more symp toms of ADHD and im - pair ment on the teacher screen. Ad di tional re search is needed to un der stand why so many chil dren remain symp to matic dur ing treat ment. Al though the prob lems with case defi ni tion and de vel op ing a re li able screening ap proach are sub stan tial, population- based epi de mologic studies of ADHD are fea si ble. Epi de mi ol ogic stud ies are needed to ad dress im por tant gaps in our un der - stand ing of the preva lence, risk fac tors, treat ment patterns, and life course of ADHD. Acknowledgements For help start ing: Dr C Carl son, Dr M Denckla, Dr P Frick, Dr R Klein, and Dr I Rapin. For help conducting the study: V Lang ston, K Beamon, J Al li son, S Bicknell, S Harvin, A Overby, D Dal mas, E Shel ton, and M Staf ford. For criti cal feed back: M Boh lig, Dr D Baird, Dr F Stal lone, Dr LP Row land, Dr H Solterer, and Dr AJ Wil - cox. Clinical Im pli ca tions The prevalence of DSM-IV defined attention-deficit hyperactivity disorder (ADHD) may be substantially underestimated. When DSM- IV criteria are applied systematically in a community setting, ADHD appears to be both underdiagnosed and overdiag - nosed. In a community setting, many children being treated for ADHD with stimulant medication continue to show many symptoms and impair - ment at school. This phenomenon needs to be better understood. Limi ta tions This study was conducted in 1 North Carolina county. It is not clear how generalizable the re sults are to other locales. There was possible selection bias: teachers who volunteered may have been assigned more children with behaviour problems, which would lead to overestimating prevalence. The methods used to define impairment or to combine symptoms need additional research and refinement. Funding for this study was provided by the NIEHS epidemiology branch. This study was ap proved by the John ston County School Board and by the NIEHS Institutional Re view Board. References 1. Bar kley RA. Attention- deficit hy per ac tiv ity dis or der: a hand book for di ag no sis and treat ment. New York: Guil ford; Dis cala C, Lesco hier I, Bar thel M, Li G. In ju ries to children with at ten tion deficit hy per ac tiv ity dis or der. Pe di at rics 1998;102: Man nuzza S, Klein RG, Bessler A. Adult out come of hy per ac tive boys. Arch Gen Psy chia try 1993;50: Mof fitt TE. Ju ve nile de lin quency and at ten tion deficit dis or der; boy s de vel op men - tal tra jec to ries from age 3 to age 15. 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Ex peri men tal cross- validation of DSM-IV types of attention- deficit/hy per ac tiv ity dis or der. J Am Acad Child Ado lesc Psy chia try 1999;38: Résumé : Étude de l épidémiologie du trouble d hyperactivité avec déficit de l attention : résultats de la méthode de dépistage et de l essai pilote Ob jec tif : Dans le cadre d une vaste étude épidémi olo gique des fac teurs de risque du trou ble d hy per ac tiv ité avec défi cit de l at - ten tion (THADA), cette étude pi lote com bi nait l in for ma tion des par ents et des en seig nants pour es ti mer la préva lence du THADA chez des en fants du pri maire d un comté de la Caro line du Nord. Les méth odes mises au point pour cette étude et les pièges que nous avons ren con trés il lus trent les pro blèmes que com por tent les études du THADA dans la popu la tion. Méth odes : Nous avons utilisé un dépistage en deux étapes d a près les critères du DSM- IV. Les en seig nants ont répondu à des échelles de clas se ment du com por te ment pour tous les enfants. Nous avons en suite ad min is t ré une en tre vue té léphonique struc - tu rée aux par ents des cas po ten tiels. Nous avons procédé au dépistage au près de 362 des 424 (85 %) en fants des classes de 1 re à 5 e an née de 4 écoles. Résul tats : Selon les dé cla ra tions des par ents, 43 en fants (12 %) avaient pré cédemment reçu un di ag nos tic de THADA par un pro fes sion nel de la santé. Trente- quatre en fants (9 %) prenaient des médica ments pour le THADA. Quarante- six enfants (12,7 %) sat is fai saient aux critères d étude de cas du THADA, selon les dé cla ra tions combinées des par ents et des en seig nants. Sur les 46 cas, 18 (39 %) n a vaient pas été repérés au para vant. Ce pend ant, 8 en fants pré cédemment di ag nos tiqués ne sat is fai - saient pas aux critères de cas. Après des cor rec tions pour non- réponse, la préva lence es timée était de 16 % (95 % IC, 12 % à 20 %). Con clu sions : Ces données in diquent que la préva lence du THADA du DSM- IV a été sub stan tiel le ment sous- estimée, bien que la préva lence réelle chez cette popu la tion puisse être infé rieure au 16 % es timé ici. Les études du THADA en popu la tion gé né rale sont faisables et peu vent four nir d im por tants ren seig ne ments sur la pra tique et les modes de traite ment dans un cadre com - munautaire ainsi qu une com préhen sion élar gie de l é ti olo gie et de l évo lu tion de ce trou b le répandu.