Attention Deficit-Hyperactivity Disorder and Month of School Enrollment

Abstract

Background: Younger children in a school grade cohort may be more likely to receive a diagnosis of attention deficit-hyperactivity disorder (ADHD) than their older peers because of age-based variation in behavior that may be attributed to ADHD rather than to the younger age of the children. Most U.S. states have arbitrary age cutoffs for entry into public school. Therefore, within the same grade, children with birthdays close to the cutoff date can differ in age by nearly 1 year.

Methods: We used data from 2007 through 2015 from a large insurance database to compare the rate of ADHD diagnosis among children born in August with that among children born in September in states with and states without the requirement that children be 5 years old by September 1 for enrollment in kindergarten. ADHD diagnosis was determined on the basis of diagnosis codes from the International Classification of Diseases, 9th Revision. We also used prescription records to compare ADHD treatment between children born in August and children born in September in states with and states without the cutoff date of September 1.

Results: The study population included 407,846 children in all U.S. states who were born in the period from 2007 through 2009 and were followed through December 2015. The rate of claims-based ADHD diagnosis among children in states with a September 1 cutoff was 85.1 per 10,000 children (309 cases among 36,319 children; 95% confidence interval [CI], 75.6 to 94.2) among those born in August and 63.6 per 10,000 children (225 cases among 35,353 children; 95% CI, 55.4 to 71.9) among those born in September, an absolute difference of 21.5 per 10,000 children (95% CI, 8.8 to 34.0); the corresponding difference in states without the September 1 cutoff was 8.9 per 10,000 children (95% CI, -14.9 to 20.8). The rate of ADHD treatment was 52.9 per 10,000 children (192 of 36,319 children; 95% CI, 45.4 to 60.3) among those born in August and 40.4 per 10,000 children (143 of 35,353 children; 95% CI, 33.8 to 47.1) among those born in September, an absolute difference of 12.5 per 10,000 children (95% CI, 2.43 to 22.4). These differences were not observed for other month-to-month comparisons, nor were they observed in states with non-September cutoff dates for starting kindergarten. In addition, in states with a September 1 cutoff, no significant differences between August-born and September-born children were observed in rates of asthma, diabetes, or obesity.

Conclusions: Rates of diagnosis and treatment of ADHD are higher among children born in August than among children born in September in states with a September 1 cutoff for kindergarten entry. (Funded by the National Institutes of Health.).

Figure 1.. Differences in Diagnosis Rates of Attention Deficit–Hyperactivity Disorder (ADHD) According to Month of Birth.

Each point represents the absolute difference in the rate of ADHD diagnosis per 10,000 children between children born in a given month and children born in the following month. For example, the “Aug. vs. Sept.” marker indicates that the absolute difference in the diagnosis rate between children born in August and children born in September was 21.5 per 10,000 children (95% CI, 8.8 to 34.0). The dashed line indicates no difference. I bars indicate 95% confidence intervals. ADHD diagnosis was defined on the basis of a diagnosis code for ADHD according to the International Classification of Diseases, 9th Revision (ICD-9), on billing claims or on the basis of a prescription filled for an ADHD medication.

Figure 2.. Differences in ADHD Diagnosis Rates According to Month of Birth in States with and States without a September 1 Cutoff.

Shown are the differences in ADHD diagnosis rates between children in the 18 states with a September 1 cutoff for kindergarten entry and children in all states without a September 1 cutoff. The dashed line indicates no difference. I bars indicate 95% confidence intervals. ADHD diagnosis was defined on the basis of an ICD-9 diagnosis code for ADHD on billing claims or a prescription filled for an ADHD medication.

Figure 3.. Differences in ADHD Diagnosis Rates According to Sex and Month of Birth.

The dashed line indicates no difference. I bars indicate 95% confidence intervals. ADHD diagnosis was defined on the basis of an ICD-9 diagnosis code for ADHD on billing claims or a prescription filled for an ADHD medication.