I read in a BBC news article "There is no test for ADHD. It is diagnosed on the basis of a questionnaire, which is subjective." [http://news.bbc.co.uk/1/hi/health/6302209.stm].

I don't get it.  It's not like in a diabetic patient you sit them down and observe them over a period of time and ask them questions like, "When you don't eat for a while, do you feel unusually weak or tired?" and if they answer "yes" to a majority of the questions asked, you diagnose them with diabetes, with out any blood test, or any other type of biological test.