title: The state of asthma epidemiology: an overview of systematic reviews and their quality.
abstract: BACKGROUND
  Recently, we have published an overview of systematic reviews in allergy epidemiology and identified asthma as the most commonly reviewed allergic disease. Building on this work, we aimed to investigate the quality of systematic reviews in asthma using the AMSTAR checklist and to provide a reference for future, more in-depth assessment of the extent of previous knowledge.
  
  
  METHODS
  We included all 307 systematic reviews indexed with asthma, including occupational asthma, and/or wheeze from our previous search in PubMed and EMBASE up to December 2014 for systematic reviews on epidemiological research on allergic diseases. Topics of the included systematic reviews were indexed and we applied the AMSTAR checklist for methodological quality to all. Statistical analyses include description of lower and upper bounds of AMSTAR scores and variation across publication time and topics.
  
  
  RESULTS
  Of 43 topics catalogued, family history, birth weight, and feeding of formula were only covered once in systematic reviews published from 2011 onwards. Overall, at least one meta-analysis was conducted for all topics except for "social determinants", "perinatal", "birth weight", and "climate". AMSTAR quality scores were significantly higher in more recently published systematic reviews, in those with meta-analysis, and in Cochrane reviews. There was evidence of variation of quality across topics even, after accounting for these characteristics. Genetic factors in asthma development were often covered by systematic reviews with some evidence of unsubstantiated updates or repetition.
  
  
  CONCLUSIONS
  We present a comprehensive overview with an indexed database of published systematic reviews in asthma epidemiology including quality scores. We highlight some topics including active smoking and pets, which should be considered for future systematic reviews. We propose that our search strategy and database could be a basis for topic-specific overviews of systematic reviews in asthma epidemiology.
authors.affiliation: Institute of Epidemiology and Medical Biometry, Ulm University, Helmholtzstr. 22, 89081 Ulm, Germany.
authors.forename: Jon
authors.identifier: 0000-0001-5764-1528
authors.initials: J
authors.lastname: Genuneit
authors.affiliation: Institute of Epidemiology and Medical Biometry, Ulm University, Helmholtzstr. 22, 89081 Ulm, Germany.
authors.forename: Annina M
authors.initials: AM
authors.lastname: Seibold
authors.affiliation: Institute of Epidemiology and Preventive Medicine, University of Regensburg, Regensburg, Germany.
authors.forename: Christian J
authors.initials: CJ
authors.lastname: Apfelbacher
authors.affiliation: Department of Allergy and Clinical Immunology, 424 General Military Training Hospital, Thessaloniki, Greece.
authors.forename: George N
authors.initials: GN
authors.lastname: Konstantinou
authors.affiliation: Murdoch Children's Research Institute, University of Melbourne, Melbourne, Australia.
authors.forename: Jennifer J
authors.initials: JJ
authors.lastname: Koplin
authors.affiliation: Institute of Biomedicine and Molecular Immunology, National Research Council of Italy, Palermo, Italy.
authors.forename: Stefania
authors.initials: S
authors.lastname: La Grutta
authors.affiliation: Division of Asthma, Allergy and Lung Biology, Children's Allergies Department, King's College London, London, UK.
authors.forename: Kirsty
authors.initials: K
authors.lastname: Logan
authors.affiliation: Unit for Population-Based Dermatology Research, St John's Institute of Dermatology, King's College London and Guy's and St Thomas' NHS Foundation, London, UK.
authors.forename: Carsten
authors.initials: C
authors.lastname: Flohr
authors.affiliation: Population Health Research Institute, St George's, University of London, London, UK.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Perkin