This systematic review pulled together 12 long-term studies that diagnosed children with ADHD and then reassessed them as adults. The children were all diagnosed carefully in childhood, and the authors note most came from white, male, middle-class clinic groups who were not followed past young adulthood. The review found 41 separate estimates of how many of these children still had ADHD as adults, and the estimates ranged widely (from 4.0% to 77.0%). The main reason for that spread was how the adult diagnosis was made, not the people themselves.
Studies that relied only on what adults said about themselves, together with a strict count of six ADHD symptoms, gave the lowest persistence rates. When reports from parents or others who knew the adult were added in, the rates rose. Studies that used the review's recommended methods reported persistence of about 40% to 50%. One study, the Milwaukee sample, used the full set of recommended methods and found persistence of 40% to 44%. Dropping estimates based on self-report alone narrowed the range to 12.0% to 77.0% (18 estimates), and also dropping the strict six-symptom rule narrowed it further to 40.0% to 77.0% (10 estimates).
The authors suggest that studies measuring adult persistence collect both self and informant reports, require signs of real impairment, and use an age-adjusted symptom threshold.
Source: Sibley M, Mitchell J, Becker S. Method of adult diagnosis influences estimated persistence of childhood ADHD: a systematic review of longitudinal studies. The Lancet Psychiatry (2016). doi:10.1016/s2215-0366(16)30190-0