Attention deficit disorder, also known as attention deficit hyperactivity disorder, or ADHD, usually is diagnosed in school-age children.
The diagnosis is commonly based on observed behavior, and some experts believe it is highly subjective, essentially just an educated judgment.
Some say the condition is being over-diagnosed in the United States, exposing children unnecessarily to medication, while others argue it is not treated often enough.
Earlier studies have shown scans can detect structural differences in ADHD sufferers' brains, as well as abnormalities in brain activity, and scientists suspect that defects in genes relating to the brain chemical dopamine probably are involved.
The latest study, conducted at Massachusetts General Hospital in Boston and published in this week's issue of The Lancet medical journal, is the first to show a measurable biochemical abnormality in people with the disorder.
The method "is the most promising development I've seen in a long time in terms of our coming up with an actual physical test that could help us pin down the diagnosis of ADHD," said Dr. Edward Hallowell of Harvard Medical School, who was not connected with the research.
The Boston scientists have come up with the most direct indicator to date, measuring the biochemical balance in the brain by counting the number of dopamine transporters.
Dopamine is associated with movement, thought, motivation and pleasure. One brain cell signals another by squirting dopamine. Then the first cell mops up the released chemical with a structure called a dopamine transporter.
The researchers scanned the brains of six adults diagnosed with ADHD and 30 healthy people of the same age after injecting both groups with a chemical agent that attaches to the dopamine transporter.
The ADHD sufferers had 70 percent more dopamine transporters than their healthy counterparts.
The scientists could not tell, however, whether that was a cause or an effect of the disorder.
The increased number could either mean not enough dopamine floating is around the system or that too much is being produced, said one of the researchers, Dr. Bertha Madras, a professor of psychobiology at Harvard Medical School.
"It's very early days, but if all hyperactive adults and children show a 70 percent increase above normal in this test, I think it would de facto be considered a diagnostic," Madras said.
ADHD has been estimated to affect 3 percent to 5 percent of school-age children in the United States, and about 1 percent of children in Europe.
The disorder is characterized by impulsive behavior and difficulties in paying attention and keeping still to read, study or even watch television. Some mild fors of the symptoms are common in many children, leading to the concerns that it is diagnosed too often.
Dr. Sam Tucker, a London-based pediatrician who specializes in ADHD, said that while the test could be useful, its potential as a definitive diagnostic test is uncertain.
"Scanning is the way to go, but this alone is not going to be the whole answer," he said. "I wouldn't use this as a diagnostic because the neurochemistry is extremely complicated and we don't know how many chemicals are important. It may be two, it may be several."
The researchers agreed doctors still will have to evaluate a child's behavior to help them diagnose the disease, but said the new test could be an important tool.
By Emma Ross