CHICAGO, Sept 3 (Reuters) - More than 2 million U.S. children likely fit the diagnosis for attention deficit/hyperactivity disorder, and more than half go untreated, researchers said on Monday.

They found evidence that 8.7 percent of children aged 8 to 15 may have the disorder, which hurts their ability to maintain concentration.

Previous studies have put the rate of ADHD at anywhere between 2 percent and 26 percent of U.S. children.

Poor children were least likely to get treatment for their condition, Tanya Froelich of the Cincinnati Children's Hospital Medical Center and colleagues found.

They used data from a federal government survey of 3,000 children aged 8 to 15 experts say reflects the overall population. Projected to the U.S. population as a whole, their 8.7 percent rate would mean that 2.4 million children suffer from ADHD, the researchers said.

Of those that met ADHD criteria in the study, only 48 percent had been diagnosed by a health professional. Roughly one-third received consistent treatment with medications, the researchers reported in the Archives of Pediatric Adolescent Medicine.

One goal of the study was to firmly establish just how common ADHD really is -- an issue that prompts considerable disagreement among experts and consternation among parents.

Many groups have alleged that children are in fact being over-medicated for behavior that fits within the normal range.

"There is also broad public concern about rates of psychostimulant use for ADHD, yet those concerns are difficult to evaluate" without knowing who fits the diagnosis, the researchers wrote.

They used criteria from the DSM-IV, the latest edition of a widely used reference book for mental disorders.

"We also observed that less than half of children who met DSM-IV criteria for ADHD had reportedly had their conditions diagnosed or been treated with ADHD medications, suggesting that some children ... may not be receiving optimal interventions," the researchers wrote.

Among the other findings of the study was that girls were less likely to have the disorder recognized -- perhaps because ADHD is more common among young boys than young girls.

Also, the poorest children in the study were more likely to have ADHD -- and up to five times less likely to be treated -- which may be related to their increased risk of being born prematurely or the likelihood that they were exposed to toxic substances such as lead.

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