title: Variability in diagnostic accuracy can be estimated using simple population weighting.
abstract: OBJECTIVE
  Diagnostic accuracy of a quantitative diagnostic test at a given numeric cutoff is dependent on the type of population (e.g., chronic, referrals, or screening) under investigation. Simple weighted averages calculated from a single study sample may be used to assess variability in accuracy in different types of populations.
  
  
  METHODS
  We evaluated the accuracy of the 4-variable Modification of Diet in Renal Disease (MDRD) Study equation as a diagnostic test to separate stage 1 and 2 chronic kidney disease (>or=60 mL/min per 1.73 m(2)) from stage 3-5 (<60 mL/min per 1.73 m(2) requiring treatment to prevent progression) in a sample of 850 patients referred for determination of glomerular filtration rate (GFR). Using population weighting, we also estimated the accuracy of the MDRD equation when the GFR distribution typically found in screening situations was mimicked.
  
  
  RESULTS
  Estimated diagnostic accuracy of the MDRD equation varied substantially for different population types (sensitivity range 82%-97%, specificity 67%-93%; figures include the original MDRD study).
  
  
  CONCLUSIONS
  Reports of diagnostic accuracy should include estimates of the variability of diagnostic accuracy, using different real or tentative population distributions. Population weighting is a useful tool for this purpose.
authors.affiliation: Competence Centre for Clinical Research, Lund University Hospital, Barngatan 2, SE-221 85 Lund, Sweden. jonas.bjork@skane.se
authors.forename: Jonas
authors.initials: J
authors.lastname: Björk
authors.forename: Anders
authors.initials: A
authors.lastname: Grubb
authors.forename: Ulf
authors.initials: U
authors.lastname: Nyman
mesh_terms: D000465:Algorithms; D003404:Creatinine / Q000097:blood; D003955:Diagnostic Tests, Routine / Q000592:standards* / Q000706:statistics & numerical data; D004032:Diet; D005260:Female; D005919:Glomerular Filtration Rate; D006801:Humans; D007676:Kidney Failure, Chronic / Q000175:diagnosis* / Q000453:epidemiology; D008297:Male; D008954:Models, Biological; D044382:Population Groups; D012680:Sensitivity and Specificity; D013548:Sweden / Q000453:epidemiology
chemical_list: D003404:Creatinine