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Considering the Control Group: The Influence of Follow-Up Assessments on Asthma Symptoms. OBJECTIVE: It is unclear whether research participation effects contribute to an improvement in asthma symptoms during clinical trials in the absence of any active intervention. We examined the impact of additional follow-up surveys on caregiver-reported symptoms among control subjects in a series of randomized controlled asthma trials. METHODS: We analyzed baseline and follow-up data for children (3-10 years) with poorly controlled persistent asthma that participated as control subjects in 1 of 3 randomized trials of urban school-based asthma care (study duration: 7-10 months). We compared mean symptom-free days (SFD) per 2 weeks between baseline and final follow-up; performed bivariate regressions to explore associations between demographics and changes in SFD; and performed multivariate random-effects generalized least square regression to examine the relationship between number of follow-ups beyond baseline (range: 1-10) and changes in SFD over time. RESULTS: Five hundred and sixteen children were enrolled as controls across the 3 trials (mean age 7.5 years, 61% Black, 28% Hispanic, 81% Medicaid). Mean SFDs increased significantly from baseline to final follow-up (7.8-11.4 days, P < .001). In adjusted analyses, significant improvements in SFD were observed with all follow-up contacts in comparison with baseline. Symptom improvement showed a dose-response relationship with the number of follow-up assessments completed (1, 2-3, 4-5, and 6-10 assessments). CONCLUSIONS: Children with uncontrolled asthma who participate as controls in clinical trials experience a significant increase in SFD with additional follow-up assessments. This improvement should be considered when designing/analyzing asthma interventions, and may help guide clinical outreach efforts for underserved children with persistent asthma.