Background: Identify a group of adults with 'undiagnosed' attention deficit hyperactivity disorder (ADHD) and compare their personal and family medical histories, psychosocial profiles, functional impairment and quality of life with non-ADHD controls. Additionally, compare adults with undiagnosed and diagnosed ADHD to investigate possible reasons why the undiagnosed avoid clinical detection.

Method: ICD-9 codes for ADHD in administrative claims records and responses to a telephone-administered adult ADHD screener [the Adult ADHD Self-Report Scale (ASRS)] were used to classify approximately 21000 members of two large managed health-care plans as 'undiagnosed' (no coded diagnosis; ASRS positive) or 'non-ADHD' controls (no coded diagnosis; ASRS negative). Patients identified as 'undiagnosed' ADHD were compared with samples of non-ADHD controls and 'diagnosed' ADHD patients (ICD-9 coded ADHD diagnoses) on the basis of demographics, socio-economic status, past and present mental health conditions, and self-reported functional and psychosocial impairment and quality of life.

Results: A total of 752 'undiagnosed' ADHD subjects, 199 'non-ADHD' controls and 198 'diagnosed' ADHD subjects completed a telephone interview. Overall, the 'undiagnosed' ADHD cohort demonstrated higher rates of co-morbid illness and greater functional impairment than 'non-ADHD' controls, including significantly higher rates of current depression, and problem drinking, lower educational attainment, and greater emotional and interpersonal difficulties. 'Undiagnosed' ADHD subjects reported a different racial composition and lower educational attainment than 'diagnosed' ADHD subjects.

Conclusion: Individuals with 'undiagnosed' ADHD manifest significantly greater functional and psychosocial impairment than those screening negative for the disorder, suggesting that ADHD poses a serious burden to adults even when clinically unrecognized.