Two studies published recently suggest there could be something wrong with the way ADHD is diagnosed in young children in the US, one found that nearly 1 million kids are potentially misdiagnosed just because they are the youngest in their kindergarten year, with the youngest in class twice as likely to be on stimulant medication, while the other study confirmed that whether children were born just before or just after the kindergarten cutoff date significantly affected their chances of being diagnosed with ADHD.
Papers on both studies by US researchers are in press, to be published in the Journal of Health Economics, the first being a corrected proof that was first available online in June, and the other appeared online on 4 August.
In the first paper, Dr Todd Elder, assistant professor of economics at Michigan State University, looked at a sample of nearly 12,000 children from the Early Childhood Longitudinal Study Kindergarten Cohort, which is funded by the National Center for Education Statistics. He analysed the difference in ADHD diagnosis and medication rates between the youngest and the oldest children in a kindergarten grade.
He found that the youngest children were significantly more likely to be diagnosed with ADHD and to be prescribed behavior-modifying stimulants such as Ritalin than their older classmates. He told the press that the "smoking gun" was that the diagnoses depended on the children's age relative to classmates and the teacher's perceptions of whether they had symptoms.
"If a child is behaving poorly, if he's inattentive, if he can't sit still, it may simply be because he's 5 and the other kids are 6."
"There's a big difference between a 5-year-old and a 6-year-old, and teachers and medical practitioners need to take that into account when evaluating whether children have ADHD," he urged.
Elder said medicating such children inappropriately was a cause for concern not just because of the effect of long term stimulant use on their health but also because it costs a lot of money: he estimated about 320 to 500 million US dollars is being wasted on unnecessary medication of young children for ADHD, of which 80 to 90 million is funded by Medicaid.
From his analysis, Elder found that the youngest kindergarten kids were 60 per cent more likely to be diagnosed with ADHD than the oldest in the same grade, and also, by the time those groups reached the fifth and eighth grades, the youngest were more than twice as likely to be on prescription stimulants.
Elder estimated that overall in the US, the misdiagnosis rate is about 1 in 5, that is around 900,000 of the 4.5 million children currently diagnosed with ADHD have been misdiagnosed.
Like the researchers in the second study, Elder used kindergarten eligibility cutoff dates to distinguish between the youngest and the oldest kids in a grade. While this date differs among states in the US, the most commonly used one is that used by 15 states to rule that kids must be 5 years old on or before 1st September to be eligible for kindergarten.
He found the same definitive pattern both in the case of individual states and when he compared across states.
Michigan for example has a cutoff date of 1st December for kindergarten attendance. Elder found higher rates of diagnosed ADHD among Michigan kids born on 1st December than born on 2nd December. Those born on the 1st December would have been the youngest in their grade, while those born on the 2nd, just one day later, because of the cutoff date, would have enrolled a year later and therefore been among the oldest in their grade.
Elder remarked that even though these kids were only born a day apart, they were assessed differently because they were being compared with classmates of a different age set.
Looking across states, Elder gave the example of Illinois and Michigan. In Illinois, where the cutoff date for kindergarten is 1st September, August-born kids were more likely to have been diagnosed with ADHD than Michigan kids born in August of the same year.
Elder's study defined a diagnosis of ADHD as including evidence of multiple symptoms, including inattention and hyperactivity sustained for six months or more observed in two settings, for instance the home and school, before the age of seven.
Although a mental health professional performs the diagnosis, the opinions of teachers often influence whether a child is sent for evaluation in the first place, said Elder.
"Many ADHD diagnoses may be driven by teachers' perceptions of poor behavior among the youngest children in a kindergarten classroom," said Elder, but the "symptoms" that teachers perceive may "merely reflect emotional or intellectual immaturity among the youngest students".
ADHD is the most commonly diagnosed behavioral disorder for kids in the United States, and currently there are no neurological markers for ADHD (such as a blood test for example). Experts disagree on how common it is, hotting up public debate about whether it is under- or over-diagnosed, said Elder.
In the second paper, researchers at North Carolina (NC) State University, Notre Dame and the University of Minnesota drew very similar conclusions to those of Elder's study.
Co-author Dr Melinda Morrill, a research assistant professor of economics at NC State, told the press that:
"The question we asked was whether children who are relatively young compared to their classroom peers were more likely to be diagnosed with ADHD."
Morrill and colleagues looked at kids born just before the kindergarten eligibility cutoff date and those born shortly after and found large discrepancies in rates of ADHD diagnosis and treatment based on small differences in birth dates.
For the study they analyzed data from two national health surveys and a national database of private health insurance claims. The data covered several periods between 1996 and 2006.
They found that kids who were "relatively old-for-grade", that is those born just after the kindergarten cutoff date, were 25 per cent less likely to have received a diagnosis for ADHD than their the "relatively young-for-grade" peers, that is kids born just before the cutoff date.
As their premise was that children born a few days apart should have the same underlying risk of developing ADHD, finding a significant discrepancy based on small differences in age suggests the problem is inappropriate diagnosis, concluded the researchers.
"This indicates that there are children who are diagnosed (or not) because of something other than underlying biological or medical reasons," said Morrill.
"We believe that younger children may be mistakenly diagnosed as having ADHD, when in fact they are simply less mature," she added, drawing the same conclusion as Elder in the first study.
However, she wished to stress that their study is "not downplaying the existence or significance of ADHD in children".
"What our research shows is that similar students have significantly different diagnosis rates depending on when their birthday falls in relation to the school year," she pointed out.
"The importance of relative standards in ADHD diagnoses: Evidence based on exact birth dates."
Todd E. Elder
Journal of Health Economics, In Press, Corrected Proof, Available online 17 June 2010
"Measuring Inappropriate Medical Diagnosis and Treatment in Survey Data: The Case of ADHD among School-Age Children."
William N. Evans, Melinda S. Morrill, Stephen T. Parente
Journal of Health Economics, In Press, Accepted Manuscript, Available online 4 August 2010.
-- Journal of Health Economics
Michigan State University, North Carolina State University.