In this article from WebMD, John Casey writes:
Treating adult ADHD is fairly simple, but diagnosing it is difficult. Many adults realize they have ADHD when their children are diagnosed with it.
I've been re-reading Thom Hartmann's book Attention Deficit Disorder: A Different Perception, which I was fortunate to get used at a bookstore last year. I've shared my discoveries with my mom, who has been diagnosed with bipolar disorder. She seems to believe that she may have ADHD as well.
It is not uncommon for both to be diagnosed as the other. More from the article:
In June, [Dr. Lenard Adler, director of the Combined Departments of Psychiatry and Neurology Adult ADHD Program at New York University School of Medicine,] and his colleagues released a survey of how 400 primary care doctors rated their ability to diagnose adult ADHD: Nearly 50 percent of the doctors said they do not feel confident in diagnosing ADHD in adults...I took a self-assessment type of test in January/February 2002, and I scored high in certain areas, which led my psychiatrist at that time to diagnose me with ADHD.
Working with the World Health Organization, Adler and his colleagues developed an ADHD screening test asking patients to answer six questions about their behavior. If they score high, they take a more precise, 18-item symptoms checklist test.
“The advantage of the self-test is that it is patient friendly and standardized,” says Adler. “We hope this will take a great deal of guesswork out of diagnosis.”
The ADHD self-assessment test, which is copyrighted by the WHO, is available online at www.med.nyu.edu/Psych/training/adhd.html.