According to DSM-IV, there are three types of Attention Deficit Hyperactivity Disorder (ADHD): Primarily Inattentive (PI), Hyperactive/Impulsive (H/I), and Combined type (CT). The DSM criteria specify that 6 out of 9 PI symptoms and/or 6 out of 9 H/I symptoms are required to qualify for a diagnosis of ADHD. The fact that any 6 symptoms from the PI list or from the H/I list can meet the diagnostic criterion reflects a “polythetic” model of ADHD.
According to the polythetic model, ADHD symptoms are interchangeable in the sense that various combinations of at least 6 symptoms-rather than the presence of any particular symptoms-define ADHD. An implication of the polythetic model is that ADHD symptoms form a continuum, whereby the criterion of 6 symptoms represents one point on the continuum, rather than marking a categorical boundary between no disorder vs. disorder.
Lubke et al. (2009) tested the degree to which continuous vs. categorical models of attention problems agree with DSM-IV diagnoses of ADHD among Dutch twin boys. Factor analytic (continuous) and latent class (categorical) models were fitted to mothers’ CBCL ratings of 8,079 boys at age 7, 5,278 boys at age 10, and 3,139 boys at age 12.
ADHD diagnoses were made from Diagnostic Interview Schedule for Children (DISC) interviews with the mothers of 449 boys at age 7, 336 at age 10, and 331 at age 12. DSM diagnoses agreed with continuous factor models better than with categorical models at all three ages: 100% of boys diagnosed as ADHD CT at ages 7 and 10, and 94.6% diagnosed as ADHD CT at age 12 obtained CBCL scores classified as high on attention problems; 100% of boys diagnosed as ADHD H/I at ages 7 and 12 and 83.1% diagnosed as ADHD H/I at age 10 likewise obtained high CBCL attention problems scores.
However, at all three ages, most boys diagnosed as ADHD PI obtained moderately high CBCL attention problems scores, while a minority obtained high scores. The authors concluded that “a clinician will benefit from knowing that APs (attention problems) exist on a severity continuum, thus presenting a clear invitation to develop evidence-based interventions that aim toward diminishing the severity of the symptoms within the continuum” (p. 1092).
The authors also drew an analogy with hypertension: a diastolic pressure of 100 warrants treatment, whereas a diastolic pressure of 80 does not warrant treatment, but obviously does not indicate an absence of blood pressure. Analogously, high scores on a continuum of attention problems may warrant treatment, whereas low scores do not warrant treatment, although they do not necessarily indicate a total absence of attention problems.
Reference: Lubke, G.H., Hudziak, J.J., Derks, E.M., van Bijsterveldt, T.C.E.M., & Boomsma. D.I. (2009). Maternal ratings of attention problems in ADHD: Evidence for the existence of a continuum. Journal of the American Academy of Child and Adolescent Psychiatry, 48, 1085-1093.