Considering the Control Group: The Influence of Follow-Up Assessments on Asthma Symptoms

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.

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.

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).

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.