The search for biomarkers for attention deficit/hyperactivity disorder.

The main characteristic of attention deficit/hyperactivity disorder (ADHD) is a persistent pattern of inattention and/or hyperactivity-impulsivity which is more frequent and severe than is usually expected in individuals at a comparable level of development. ADHD is estimated to affect approximately 5.29% of school-aged children and is therefore the most common childhood onset psychological disorder. A conservative estimate of the annual societal cost of illness for ADHD in childhood and adolescence is USD 42.5 billion in the U.S. alone. Global sales of ADHD medicines could reach USD 4.3 billion by 2012. Despite the prevalence, high heritability and costs of ADHD, biological markers do not exist. Such biomarkers are in high demand as they would help eliminate the subjective diagnoses based on interviews and potentially allow for earlier diagnosis and personalized medicine. Lack of markers likely stems from several factors that complicate ADHD research and the assessment of pharmacological responses. This review analyses complicating factors in defining ADHD phenotype and etiology, identifying specific diagnostic markers and the difficulties in the assessment of pharmacogenomic markers. The dopamine transporter (DAT1) genotype and methylphenidate (MPH) response are detailed as an example of a biomarker. A recent report of a novel ADHD gene and its possible role as a biomarker is explored. Finally, suggestions for strategies and study designs for future research for the definition of effective ADHD biomarkers are made.