title: Acute appendicitis: meta-analysis of diagnostic performance of CT and graded compression US related to prevalence of disease.
abstract: OBJECTIVE
  This study was a head-to-head comparison of graded compression ultrasonography (US) and computed tomography (CT) in helping diagnose acute appendicitis with an emphasis on diagnostic value at different disease prevalences, commonly occurring in various hospital settings.
  
  
  METHODS
  MEDLINE, EMBASE, CINAHL, and Cochrane databases were searched from January 1966 to February 2006. Prospective trials were selected if they (a) compared graded compression US and CT in the same patient population; (b) included more than 10 patients, otherwise, the study was considered a case report; (c) evaluated mainly adults or adolescents; (d) used surgery and/or clinical follow-up as reference standard; and (e) reported data to calculate 2 x 2 contingency tables for graded compression US and CT. Estimates of sensitivity, specificity, and positive and negative likelihood ratios (LRs) for US and CT were calculated. Posttest probabilities after CT and US were calculated for various clinically relevant prevalences.
  
  
  RESULTS
  Six studies were included, evaluating 671 patients (mean age range, 26-38 years); prevalence of acute appendicitis was 50% (range, 13%-77%). Positive LR was 9.29 (95% confidence interval [CI]: 6.9, 12.6) for CT and 4.50 (95% CI: 3.0, 6.7; P = .011) for US, yielding posttest probabilities for positive tests of 90% and 82%, respectively. Negative LR was 0.10 (95% CI: 0.06, 0.17) for CT and 0.27 (95% CI: 0.17, 0.43) for US (P = .013), resulting in posttest probabilities of 9% and 21%, respectively. Posttest probabilities for positive tests were markedly decreased at lower prevalences.
  
  
  CONCLUSIONS
  In head-to-head comparison studies of diagnostic imaging, CT had a better test performance than did graded compression US in diagnosing appendicitis. Ignoring the relationship between prevalence (pretest probability) and diagnostic value may lead to an inaccurate estimation of diagnostic performance.
authors.affiliation: Department of Radiology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, Suite G1-227, 1105 AZ Amsterdam, the Netherlands. a.vanranden@amc.uva.nl
authors.forename: Adrienne
authors.initials: A
authors.lastname: van Randen
authors.forename: Shandra
authors.initials: S
authors.lastname: Bipat
authors.forename: Aeilko H
authors.initials: AH
authors.lastname: Zwinderman
authors.forename: Dirk T
authors.initials: DT
authors.lastname: Ubbink
authors.forename: Jaap
authors.initials: J
authors.lastname: Stoker
authors.forename: Marja A
authors.initials: MA
authors.lastname: Boermeester
mesh_terms: D000208:Acute Disease; D000293:Adolescent; D000328:Adult; D001064:Appendicitis / Q000175:diagnosis* / Q000453:epidemiology; D005260:Female; D006801:Humans; D008297:Male; D015995:Prevalence; D011336:Probability; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed; D014463:Ultrasonography / Q000379:methods*