title: The indication area of a diagnostic test. Part I--discounting gain and loss in diagnostic certainty.
abstract: OBJECTIVE
  Test performance is conventionally expressed by gain in diagnostic certainty. We propose net diagnostic gain and indication area as more appropriate measures of test performance; then, the loss in certainty due to misclassification and the information of "no test" would be performed are taken into account.
  
  
  METHODS
  A decision analytical model was developed in which two alternative strategies were compared: testing and no testing. Correct diagnostic test results received a positive value; undesired test results received a negative value. Within the "no test" scenario, it was assumed that physicians are more prone to treat as the probability of disease is higher.
  
  
  RESULTS
  Discounting gain and loss in diagnostic certainty results in a concave function of the prior. The indication area is the range of priors with a net diagnostic gain; testing is deleterious beyond this range. The net diagnostic gain reaches a maximum at a specific prior. A freely available Web site-based calculator was developed for easy calculation of the indication area and the maximum diagnostic gain for each combination of sensitivity and specificity.
  
  
  CONCLUSIONS
  Medical testing is not indicated when the prior disease probabilities are low (as to screening for a condition) or high (for diagnostic confirmation).
authors.affiliation: Department of Radiotherapy, Academic Medical Center (AMC), University of Amsterdam, Meibergdreef 9, Amsterdam 1105 AZ, The Netherlands. Electronic address: l.stalpers@amc.nl.
authors.forename: Lukas J A
authors.initials: LJ
authors.lastname: Stalpers
authors.affiliation: Department of Epidemiology, University of Maastricht, Peter Debyeplein 1, Maastricht 6229 HA, The Netherlands.
authors.forename: Patty J
authors.initials: PJ
authors.lastname: Nelemans
authors.affiliation: Radboud Institute for Health Sciences, Radboud University Medical Center, Geert Grooteplein 21, Nijmegen 6525 EZ, The Netherlands.
authors.forename: Sandra M E
authors.initials: SM
authors.lastname: Geurts
authors.affiliation: Department of Radiotherapy, Academic Medical Center (AMC), University of Amsterdam, Meibergdreef 9, Amsterdam 1105 AZ, The Netherlands.
authors.forename: Erik
authors.initials: E
authors.lastname: Jansen
authors.affiliation: Department of Radiotherapy, Academic Medical Center (AMC), University of Amsterdam, Meibergdreef 9, Amsterdam 1105 AZ, The Netherlands.
authors.forename: Peter
authors.initials: P
authors.lastname: de Boer
authors.affiliation: Radboud Institute for Health Sciences, Radboud University Medical Center, Geert Grooteplein 21, Nijmegen 6525 EZ, The Netherlands.
authors.forename: André L M
authors.initials: AL
authors.lastname: Verbeek
mesh_terms: D000328:Adult; D003955:Diagnostic Tests, Routine; D005260:Female; D006801:Humans; D008962:Models, Theoretical; D011258:Pregnancy Tests; D012680:Sensitivity and Specificity; D064878:Web Browser
keywords: Diagnostic test; Indication area; Maximum diagnostic gain; Prevalence; Sensitivity; Specificity