Asthma and subsequent school performance at age 15-16 years: A Swedish population-based sibling control study.

Asthma may negatively affect children's school performance, such as grades and exam results. Results from previous studies have shown varying results and may have suffered from confounding and other biases. We used a Swedish population-based cohort of 570,595 children with data on asthma (including severity and control) in Grades 7-8 and 9, school performance from Grade 9 (grade point sum, non-eligibility for upper secondary school and national test results) and measured confounders from national registers. We used sibling comparisons to account for unmeasured familial factors. Children with asthma and severe asthma performed slightly better in school than children without asthma when adjusting for measured confounders, but the associations were attenuated in sibling comparisons. In contrast, children with uncontrolled asthma performed slightly worse (e.g. Grade 9: β = -9.9; 95% CI -12.8 to -7.0; Cohen's d = 0.16). This association remained for uncontrolled asthma in Grade 9 in sibling comparisons (Grade 9: β = -7.7 points; 95% CI -12.6 to -2.6; Cohen's d = 0.12), but not for Grades 7-8. The attenuation of estimates when controlling for familial factors using sibling comparisons suggests that the differences were due to familial factors, rather than being causal. The remaining associations in sibling comparisons between uncontrolled asthma in Grade 9 and school performance are consistent with a causal association.