A meta-analysis of baseline characteristics in trials on mite allergen avoidance in asthmatics: room for improvement

Abstract Background Evidence regarding the clinical effectiveness of mite allergen avoidance for the treatment of asthma is lacking. In previous meta-analyses on mite allergen control, the baseline data were not discussed in detail. This study updates and extends the existing Cochrane review by Gøtzsche and Johansen (Cochrane Database of Systematic Reviews, 2008, Art. No: CD001187), with a focus on baseline asthma outcomes and allergen exposures. Methods We used the existing trials in the original Cochrane review and included newly published studies. The baseline data for the mite allergen load from the mattress, the standardized asthma symptom score (ASS), the forced expiratory volume in 1 s percentage of predicted (FEV1 %pred.), and the histamine provocative concentration causing a 20% drop in FEV1 (PC20) were extracted. First, the mean values of the outcomes were calculated. The influence of the mite allergen load was examined with a random-effect meta-regression using the Metafor package in R. Results Forty-five trials were included; 39 trials reported strategies for concurrent bedroom interventions, and 6 trails reported strategies for air purification. The mite allergen load ranged from 0.44 to 24.83 μg/g dust, with a mean of 9.86 μg/g dust (95% CI 5.66 to 14.05 μg/g dust, I2 = 99.8%). All health outcomes showed considerable heterogeneity (standardized ASS mean: 0.13, 95% CI 0.08 to 0.18, I2 = 99.9%; FEV1 %pred. mean: 85.3%, 95% CI 80.5 to 90.1%, I2 = 95.8%; PC20 mean: 1.69 mg/mL, 95% CI 0.86 to 2.52 mg/mL, I2 = 95.6%). The covariate mite allergen load did not significantly influence health outcomes. Discussion This meta-analysis shows that mite avoidance studies are characterized by the inclusion of patients with rather mild to moderate asthma and with varying and sometimes negligible levels of allergen exposure. Future studies should focus on patients with severe asthma and increased levels of allergen exposure.