The attention-deficit hyperactivity disorder paradox: 2. Phenotypic variability in prevalence and cost of comorbidity.

The objective of this study was to use population-based data to estimate the cost and phenotypic variation of conditions comorbid with attention-deficit hyperactivity disorder (ADHD). The North Dakota Department of Health's Claims Database was used to estimate the administrative prevalence, rates of comorbidity, and cost of care for subjects with ADHD. The case population was 7745 children. We then examined the 10 most frequent comorbidities for a defined condition in the International Classification of Diseases-9th edition (ICD-9). The observed prevalence of general health conditions was 12 to 70% less than expected. The presence of a comorbid condition resulted in increases in costs of care of dollar 381 to dollar 731 per case per year. The observed prevalence of comorbid mental health conditions was 97 to 5286% higher than expected. The prevalence of conditions comorbid with ADHD varies widely, with rates of general medical conditions decreased and rates of most mental disorders increased. The costs for comorbidity with general health conditions were offset by large decreases in prevalence rates compared with controls. We identified an ADHD paradox: decreased rates of comorbid general health conditions and increased rates of comorbid mental disorders. Further research on comorbidity in ADHD is required.