The prevalence of ADHD in the general unscreened school-age population was estimated from 4 percent to 12 percent. In the general, unscreened, school-age population, prevalence of ADHD co-occurring with other disorders was estimated to be high, based on results of four studies. Of children diagnosed with ADHD, approximately 35 percent also qualified for a diagnosis of oppositional defiant disorder, 28 percent qualified for a diagnosis of conduct disorder, 26 percent qualified for a diagnosis of anxiety disorder, and 18 percent also had a depressive disorder, and 12 percent had learning disabilities. Prevalence of ADHD in a pediatric clinic setting varied widely, with few studies available for analysis. The prevalence of comorbid ADHD in a pediatric clinic setting also varied too widely to draw useful conclusions.
Studies of ADHD-specific rating scales showed that the Conners Rating Scale of 1997 is highly effective for discriminating between children with ADHD and normal controls. The studies reviewed could not be used to derive conclusions regarding the effectiveness of broad-band rating scales in distinguishing children with significant problems from children without significant problems. Evidence does not support the use of tests of such as lead levels, thyroid function brain imaging, EEG, and neurological screening to screen children suspected of having ADHD.