title: Systematic Review on the Definition of Allergic Diseases in Children: The MeDALL Study.
abstract: BACKGROUND
  During the last decades, a large number of phenotypes and disease classifications of allergic diseases have been proposed. Despite the heterogeneity across studies, no systematic review has been conducted on phenotype classification and the criteria that define allergic diseases. We aimed to identify clinically expressed, population-based phenotypes of allergic diseases and their interrelationships, to explore disease heterogeneity and to evaluate the measurements employed in disease diagnosis.
  
  
  METHODS
  We conducted a search of MEDLINE up to December 2012, to identify relevant original studies published in the English language that examine at least one objective of this systematic review in subjects aged 0-18 years. The screening of titles and abstracts and the extraction of data were conducted independently by two reviewers.
  
  
  RESULTS
  From a total of 13,767 citations, 197 studies met the criteria for inclusion, with 54% being cohort studies. Allergic diseases were studied as a single entity in 55% (109/197) of the studies or in the context of multimorbidity in 45%. Asthma accounted for 81.7% of the studies examining single diseases. Overall, up to 33 different phenotypes of allergic disease were reported. Transient early, late-onset and persistent wheeze were the most frequently reported phenotypes. Most studies (78%) used questionnaires. The skin-prick test was the preferred measurement of sensitization (64%). Spirometry and bronchial hyperresponsiveness were assessed in one third of the studies, peak flow rate in 8.6% and disease severity in 35%.
  
  
  CONCLUSIONS
  Studies reporting phenotypes of allergic diseases in children are highly heterogeneous and often lack objective phenotypical measures. A concerted effort to standardize methods and terminology is necessary.
authors.affiliation: Centre for Research in Environmental Epidemiology (CREAL), Barcelona, Spain.
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mesh_terms: D002648:Child; D006801:Humans; D006967:Hypersensitivity / Q000145:classification* / Q000175:diagnosis / Q000503:physiopathology; D010641:Phenotype