DSM IV Criteria for Diagnosis of AD/HD
In brief, the current diagnostic criteria include:
-At least 6 of 9 symptoms of inattention AND/OR
-At least 6 of 9 symptoms of hyperactivity and/or impulsivity
-Presence of symptoms prior to age 7
-Presence of symptoms in at least two different settings (e.g., school and home)
-Clear evidence of significant impairment in several arenas
-Symptoms cannot be better explained by another medical or
psychiatric disorder
-Adolescents and adults who no longer meet full criteria may be described as �in partial remission�.
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According to the DSM-IV TR, there are three subtypes:

-Attention-Deficit Hyperactivity Disorder, Combined Type

-Attention-Deficit Hyperactivity Disorder, Predominantly Inattentive Type

-Attention-Deficit Hyperactivity Disorder, Predominantly Hyperactive-Impulsive Type

The DSM-V is expected to address a variety of diagnostic issues that are currently being debated among those in the field. In particular, it is anticipated that the DSM-V coming out in 2011 will expand and clarify the diagnostic criteria by setting clear guidelines for diagnosing adults as well as addressing age of onset. The DSM-V is also likely to layout expanded guidelines for subtypes
My oberservation is that as anything in medicine, there is nothing exact and it can only be an attempt to classify. The closer/ better the classification, the better one can treat.