Investigators at Oregon Health and Science University and other centers attempt to refine subtyping of childhood ADHD by using biologically based behavioral temperament types. Groups were validated using 3 external validators: cardiac measures of respiratory sinus arrhythmia, CNS functioning via functional MRI, and clinical outcomes at 1-year follow-up. Three novel types of ADHD were recognized: mild (normative emotion regulation), surgent (extreme levels of positive approach-motivation), and irritable (extreme levels of negative emotionality, anger, and poor soothability). These types were stable over time and showed unique patterns of cardiac physiological response, resting-state functional brain connectivity, and clinical outcomes. This biologically informed temperament-based typology is thought to provide a superior description of heterogeneity in the ADHD population than any current classification. 
COMMENTARY. The use of a combination of biological markers may help to reduce heterogeneity and to identify homogeneous phenotypes of ADHD. A consensus report of the World Federation of Societies of Biological Psychiatry (WFSBP) task force on biological markers and the World Federation of ADHD determined in 2012 that no reliable ADHD biomarker had been described to date, but some promising candidates (e.g. olfactory sensitivity, substantial echogenicity) exist. The development of ADHD markers is hindered by sample heterogeneity due to etiological and phenotypic complexity and age-dependent co-morbidities .