How should an incident case of atopic dermatitis be defined? A systematic review of primary prevention studies

Background: Eczema prevention is now an active area of dermatologic and allergy research. Defining an incident case is therefore a prerequisite for such a study. Objective: We sought to examine how an incident case of atopic dermatitis was defined in previous atopic dermatitis prevention studies in order to make recommendations on a standard definition of new atopic dermatitis cases for use in future prevention trials. Methods: We conducted a systematic review of controlled interventional atopic dermatitis prevention studies by using searches of MEDLINE and Cochrane databases for studies published from 1980 to the end of January 2011. Studies that included atopic dermatitis as a secondary outcome, such as asthma prevention trials, were included. Results: One hundred two studies were included in the final analysis, of which 27 (26.5%) did not describe any criteria for defining an incident case of atopic dermatitis. Of the remaining 75 studies with reported disease criteria, the Hanifin-Rajka criteria were the most commonly used (28 studies). A disease definition unique to that particular study (21 studies) was the second most commonly used disease definition, although the sources for such novel definitions were not cited. Conclusions: The results from this systematic review highlight the need for improved reporting and standardization of the definition used for an incident case in atopic dermatitis prevention studies. Most prevention studies have used disease definitions such as the Hanifin-Rajka criteria that include disease chronicity. While acceptable for cumulative incidence outcomes, inclusion of disease chronicity precludes the precise measurement of disease onset. We propose a definition based on existing scientific studies that could be used in future prospective studies.