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The importance of environmental allergen avoidance in allergic rhinitis is widely promoted although its true clinical value is unclear. This multicentre, randomised, double blind, placebo controlled trial examined the effects of house dust mite impermeable covers on symptom control in a group of Dutch patients with allergic rhinitis over a 1 year period (114 patients in treatment group, 118 in control group). Subjects were assessed on a rhinitis specific visual analogue scale, a daily symptom scale, nasal allergen provocation response, and levels of Dermatophagoides pteronyssinus (Der p1) and D farinae (Der f1) in domestic dust samples. Despite a clear reduction in the concentration of these allergens in the treatment group at 12 months (1.29 v 4.84 μg/g dust in the control group, p<0.001), the use of mite proof covers failed to improve clinical indices of allergic rhinitis in house dust mite sensitised individuals.
The negative findings in this study indicate that strategies aimed solely at reducing specific allergen exposure are of limited use in managing symptomatic allergic rhinitis. Heterogeneous patient predisposition and provocation by other environmental agents are among the reasons cited for the lack of efficacy of these devices.