title: External validity of randomised controlled trials in asthma: to whom do the results of the trials apply?
abstract: BACKGROUND
  Asthma is a heterogeneous disease with a wide range of clinical phenotypes, not all of which may be encompassed in the subjects included in randomised controlled trials (RCTs). This makes it difficult for clinicians to know to what extent the evidence derived from RCTs applies to a given patient.
  
  
  OBJECTIVE
  To calculate the proportion of individuals with asthma who would have been eligible for the major asthma RCTs from the data of a random community survey of respiratory health.
  
  
  METHODS
  A postal survey was sent to 3500 randomly selected individuals aged 25-75 years. Respondents were invited to complete a detailed respiratory questionnaire and pulmonary function testing. Participants with current asthma were assessed against the eligibility criteria of the 17 major asthma RCTs cited in the Global Initiative for Asthma (GINA) guidelines.
  
  
  RESULTS
  A total of 749 participants completed the full survey, of whom 179 had current asthma. A median 4% of participants with current asthma (range 0-36%) met the eligibility criteria for the included RCTs. A median 6% (range 0-43%) of participants with current asthma on treatment met the eligibility criteria.
  
  
  CONCLUSIONS
  This study shows that the major asthma RCTs on which the GINA guidelines are based may have limited external validity as they have been performed on highly selected patient populations. Most of the participants with current asthma on treatment in the community would not have been eligible for these RCTs.
authors.affiliation: Medical Research Institute of New Zealand, PO Box 10055, Wellington, New Zealand.
authors.forename: Justin
authors.initials: J
authors.lastname: Travers
authors.forename: Suzanne
authors.initials: S
authors.lastname: Marsh
authors.forename: Mathew
authors.initials: M
authors.lastname: Williams
authors.forename: Mark
authors.initials: M
authors.lastname: Weatherall
authors.forename: Brent
authors.initials: B
authors.lastname: Caldwell
authors.forename: Philippa
authors.initials: P
authors.lastname: Shirtcliffe
authors.forename: Sarah
authors.initials: S
authors.lastname: Aldington
authors.forename: Richard
authors.initials: R
authors.lastname: Beasley
mesh_terms: D000328:Adult; D000368:Aged; D001249:Asthma / Q000628:therapy*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D016032:Randomized Controlled Trials as Topic / Q000592:standards*; D015203:Reproducibility of Results; D012129:Respiratory Function Tests / Q000379:methods