title: Influence of population prevalences on numbers of false positives: an overlooked entity.
abstract: BACKGROUND
  Disease prevalence alters the number of true positives (TP), true negatives (TN), false negatives (FN), and false positives (FP), even if the sensitivity and specificity of a test stays the same.
  
  
  METHODS
  We illustrate this using data for the detection of suspected acute pulmonary embolism (PE) from the Prospective Investigation of Pulmonary Embolism Diagnosis II (PIOPED II). We chose PE because of the clinical significance of the disease, the low prevalence of PE in the patient population being tested with CTPA with the widespread adoption of CTPA, and the serious clinical consequences of anticoagulation therapy in FP patients.
  
  
  RESULTS
  Based on PIOPED II data (sensitivity 83%, specificity 96%), at a disease prevalence of approximately 5%, the number of FP patients is greater than the number of TP patients. Scaled to the US population, at a disease prevalence of 5%, there would be 139,800 FPs and 3,356,200 TNs. Assuming a mortality rate of 0.5% and a 3.0% rate of major bleeding secondary to anticoagulation therapy for well-controlled patients, if all FP patients received anticoagulation, there would be 699 deaths and 4194 major bleeding complications.
  
  
  CONCLUSIONS
  At a prevalence of approximately 5% for PE, the number of FPs approaches or is greater than the number of TPs for CTPA for the detection of suspected acute PE. Patients with FP results may receive unnecessary, potentially harmful treatment with anticoagulation therapy. Population prevalence of disease needs to be taken into account along with the diagnostic accuracy of a test, because this may significantly affect downstream patient outcomes.
authors.affiliation: Department of Radiology, Division of Cardiothoracic Radiology, University of Michigan Medical Center, Ann Arbor, 48109-5302, USA. pcronin@med.umich.edu
authors.forename: Paul
authors.initials: P
authors.lastname: Cronin
authors.forename: Aine Marie
authors.initials: AM
authors.lastname: Kelly
mesh_terms: D000792:Angiography; D000925:Anticoagulants / Q000627:therapeutic use; D002986:Clinical Trials as Topic; D003657:Decision Making; D005189:False Positive Reactions; D006801:Humans; D015995:Prevalence; D011655:Pulmonary Embolism / Q000000981:diagnostic imaging* / Q000188:drug therapy / Q000453:epidemiology*; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed; D014481:United States / Q000453:epidemiology; D019564:Unnecessary Procedures
chemical_list: D000925:Anticoagulants