The importance of an accurate diagnosis
Two criteria are of paramount importance in arriving at an accurate diagnosis of adult ADHD: the age of onset and the presence of symptoms in more than one setting. When only one of these two criteria is present, i.e. the symptoms are of relatively recent onset or the behaviors occur only at home, a differential diagnosis must be considered. In this case, it may be said the individual suffers from "symptoms of adult ADHD" without qualifying for a full clinical diagnosis.
The first criterion mandates that the onset of the major symptoms of attention deficit (with or without hyperactivity) must have been observed prior to the age of 12. Given the notorious unreliability of self-reports by individuals who may be affected by this condition, it is important to verify the existence of the symptoms with one or more individuals (parents, older siblings) who may have directly observed the patient around the age of onset.
The second criterion mandates that the major symptoms of adult ADHD must occur in more than one setting to warrant a full clinical diagnosis. For example, the symptoms must occur at home and at work, or at home and in school. When the symptoms appear only in one setting (e.g., at home) and seem to disappear in other settings (e.g., at work), a full clinical diagnosis of adult ADHD is not warranted and other causes of the dysfunctional behaviors must be assumed and investigated.