The youngest children in their school grade are more likely to be diagnosed with attention-deficit/hyperactivity disorder (ADHD) than their slightly older peers in the same grade, a new study finds.
Researchers looked at ADHD diagnoses in nearly 1 million children in British Columbia, where the cutoff date for entering school in any year is Dec. 31. In other words, children born in January are the oldest in their grade; children born just before the cutoff in December are the youngest.
They found that children born in December were 39 percent more likely to be diagnosed with ADHD, and 48 percent more likely to be treated with medication for the condition compared with children born in January.
ADHD is diagnosed based on children's behavior; there is no objective test for the condition.
"Our study suggests younger, less mature children are inappropriately being labeled and treated," study researcher Richard Morrow, of the University of British Columbia, said in a statement. "It is important not to expose children to potential harms from unnecessary diagnosis and use of medications."
The new findings are line with those of two previous studies, and were published today (March 5) in the Canadian Medical Association Journal.
ADHD in children
ADHD is the most common neurobehavioral disorder in children. As of 2007, 9.5 percent of U.S. children had been diagnosed with ADHD, according to the Centers for Disease Control and Prevention (CDC).
The new research included children who were between the ages of 6 and 12 at any point during the 11-year period between 1997 and 2008.
Of the approximately 39,000 boys in the study born in December, 7.4 percent were diagnosed with ADHD, while among same number of boys born in January, 5.7 percent were diagnosed, according to the study.
Of the 37,000 girls born in December, 2.7 percent were diagnosed with ADHD, whereas 1.6 percent of girls born in January were labeled as having the condition.
The researchers noted that the percentage of children diagnosed and treated for ADHD increased gradually over the study, and peaked in the most recent years. An increase has also been observed in the U.S. — according to the CDC, rates of ADHD diagnoses increased 5.5 percent per year between 2003 and 2007.
The new findings are in line with those of a 2010 study published in the Journal of Health Economics. That study found that the oldest children in a grade were 25 percent less likely to be diagnosed with ADHD than the youngest.
Those researchers said a children's maturation may play a role in ADHD diagnoses.
"We believe that younger children may be mistakenly diagnosed as having ADHD, when in fact they are simply less mature," study researcher Melinda Morrill, of North Carolina State University, said in a statement at the time.
What an incorrect diagnosis may mean
Medication to treat ADHD can have negative health effects in children, such as sleep disruption, increased risk of cardiovascular problems and slower growth rates, the researchers of the new study wrote in their findings. However, one recent study of 1.2 million children found no increased risk of heart problems associated with drugs commonly use to treat ADHD.
An ADHD diagnosis may affect a child's social life as well, the researchers of the new study said, as teachers and parents might treat children differently, and children may develop negative ideas about themselves.
The findings, along with the fact that there is no objective test to diagnose ADHD, "strongly suggest caution be taken in assessing children for this disorder and providing treatment," the researchers wrote in their conclusion.
The risk of misdiagnosing a child with the condition might be lowered by placing a greater emphasis on children's behavior outside of school, they wrote.
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