Replication of the latent class structure of attention-deficit/hyperactivity disorder (ADHD) subtypes in a sample of Australian twins

Background: Previous efforts to subtype Attention-Deficit/Hyperactivity Disorder (ADHD) using latent class analysis (LCA) applied to DSM-IV symptom profiles of adolescent female twins from Missouri (USA) have identified distinct classes within the domains of inattention, hyperactivity-impulsivity and combined-type problems. The objective of the current report is to determine if the latent class structure of ADHD subtypes can be replicated in a culturally distinct sample of female and male Australian twins. Method: LCA was applied to parent-report DSM-IV ADHD symptom profiles of N=2,848 child and adolescent Australian twins and compared to North American findings. Separate models were fitted for females (N = 1,432) and males (N = 1,416). Results: The most congruent latent ADHD classes across samples included a non-symptomatic class, three mild-moderate and two severe classes. Also present within samples was a rare hyperactive-impulsive class and a unique class, the structure of which was idiosyncratic across samples. Mean symptom endorsement and individual symptom endorsement probabilities for each of the stable classes were similar across samples. Consistent with previous findings, there was substantial overlap between the DSM-IV inattentive and combined subtypes with the severe inattentive and severe combined latent classes. However, DSM-IV inattentive and combined subtypes were distributed over several latent classes in each sample, and a substantial proportion of individuals with no DSM-IV diagnosis were also assigned to these severe classes. Conclusions: Results from LCA using an Australian twin sample replicate six of the eight latent class subtypes previously reported using Missouri female twins and extend the findings to male twins. LCA and DSM-IV systems of ADHD classification identify different phenotypic groups, and the basis of this disparity merits further investigation.