(Health.com) -- Children who are the youngest in their class are more likely than their older classmates to be diagnosed and given medication for attention-deficit hyperactivity disorder (ADHD) -- suggesting that immaturity may be part of the problem, not ADHD.
The finding is from a study of more than 900,000 Canadian children aged 6 to 12, and it dovetails with two U.S. studies that found the same thing in 2010.
In fact, the youngest boys were 30% more likely than their oldest classmates to get an ADHD diagnosis, and the youngest girls had a 70% greater chance, according to the study conducted by Dr. E. Jane Garland, a child and adolescent psychiatrist at the University of British Columbia in Vancouver and colleagues.
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"The younger children in a grade were significantly more likely to be diagnosed, labeled, and treated with medication for what in some of them must simply be immaturity," Garland says.
The researchers looked at children born in December, the month before the cutoff date for starting school (in this case, December 31), who were therefore the youngest in their class. They compared those children to youngsters born in January, who missed the cutoff date and so were the oldest in their class -- almost a full year older than those with December birthdays.
Of the boys born in December, 7.4% were diagnosed with ADHD and 6.2% were given medication. In contrast, only 5.7% of boys born in January were diagnosed with ADHD and 4.4% were given ADHD medication. Similarly, 2.7% of girls with December birthdays were diagnosed with ADHD and 1.9% were given ADHD medication, while 1.6% of girls born in January were diagnosed with ADHD and 1.1% were given medication.
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As for children born in other months, the younger they were relative to their classmates, the more likely they were to be diagnosed with and treated for ADHD.
The study, published in the Canadian Medical Association Journal, included 937,943 children who were 6 to 12 years old between December 1997 and November 30, 2008, representing all children in this age group in the province of British Columbia.
"It definitely looks like it's a real effect, we now have three studies, and it would be good to know more about it," says Joel Nigg, PhD, a professor of psychiatry and behavioral neuroscience at Oregon Health & Science University in Portland. Nigg studies ADHD, but did not take part in Garland's research.
Diagnosing ADHD can be tricky. For one thing, problems with attentiveness and hyperactivity fall along a continuum, says Garland.
"The symptoms of ADHD are very nonspecific," she says. "If someone is tired or they haven't eaten breakfast, they'll be fidgety and unfocused." There's no lab test that says yes, you have ADHD, or no, you don't, she notes.
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Dr. James Perrin, the director of the division of general pediatrics at the Mass General Hospital for Children in Boston, says it takes more than one doctor's visit to get an ADHD diagnosis right. (Perrin helped write guidelines for diagnosing ADHD in general practice or family practice settings, which were published in 2000, but did not participate in Garland's study).
If ADHD is suspected, a child should have at least two doctor's visits along with his or her parents, and the child's teacher should weigh in too.
"In general, we don't think you can make the diagnosis of ADHD in a single visit in primary care," Perrin says. However, he admits, it does happen. "I think most physicians, pediatricians, family practitioners frankly want to do the right thing for their kids, there's no question that they would like to do this right, but they're also feeling pressure to move the child and the family through the system quickly."
And getting a diagnosis right is important. While ADHD medications can help children who have it, the treatment can have side effects and may affect sleep, appetite, and growth, the researchers note.
Psychiatrists estimate that about 3% to 7% of school children have ADHD, but the actual diagnosis rates may be higher, according to the Centers for Disease Control and Prevention. A 2007 survey of parents suggested that 9.5% of children (13.2% of boys and 5.6% of girls), or about 5.4 million youngsters aged 4 to 17 years, had been diagnosed with ADHD.
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"The study points out that slipshod diagnosis can be a problem, and if you're getting diagnosed with ADD just because you're immature compared to your peer group, then that's not good," says Dr. Edward Hallowell, a psychiatrist with offices in New York City and Sudbury, Massachusetts, who specializes in treating children and adults with ADHD. "If you use the proper diagnostic criteria and do a careful evaluation, the fact that a child is immature compared to a peer shouldn't matter."
Nigg agrees that the time spent diagnosing a child is probably playing a role in the relative-age effect.
"It's very difficult for children to get a full evaluation for ADHD," he says. "If you don't have time to do an extensive evaluation, you may make errors."
Many ADHD experts say the condition is probably both underdiagnosed and overdiagnosed; for example, kids living in poorer areas without access to health care may not ever be diagnosed with or treated for ADHD, even though they could benefit from it. At the same time,some parents may push for an ADHD diagnosis so their child can get extra time to complete standardized tests, even if that diagnosis isn't appropriate.
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"Some people don't believe in it and they never diagnose it, some people see it everywhere," Hallowell says. "If you have the correct diagnosis, wonderful, if you don't then that's bad and it could be you're not getting the treatment you need, or you're getting a treatment you don't need. The take home point to me is that you need to see someone good."
For Garland and her colleagues, the study suggests that more caution should be used when diagnosing ADHD. They say that taking a closer look at behavior outside the classroom might also help reduce the risk of a misdiagnosis.
"With this data now available from more than one study, it really becomes the responsibility of parents, physicians, and teachers to pay attention to the relative-age effect and not intervene with treatment and labeling of children who it's no fault of their own what time of year they were born," Garland says.
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