title: Variable performance of weaning-predictor tests: role of Bayes' theorem and spectrum and test-referral bias.
abstract: OBJECTIVE
  We examined whether variation in reported reliability of the frequency-to-tidal volume ratio (f/V(T)) in predicting weaning success is explained by spectrum and test-referral bias, as reflected by variation in pretest probability of success.
  
  
  METHODS
  Two authors extracted data from all studies on reliability of f/V(T) as a weaning predictor.
  
  
  RESULTS
  Prevalence of successful weaning in studies of f/V(T) revealed significant heterogeneity; mean success rate was 0.75. The heterogeneity and high success rate reflects occurrence of spectrum bias, suggested by the lower value of f/V(T) in subsequent studies than in the original report (77.4 vs. 89.1) and test-referral bias, suggested by lower specificity of f/V(T) in subsequent studies than in the original report (0.52 vs. 0.64). When data from studies in the ACCP Task Force's meta-analysis of studies on f/V(T) were entered into a Bayesian model with pretest probability (prevalence of success) as the operating point, observed posttest probabilities were closely correlated with values predicted by the original report on f/V(T): positive-predictive value r = 0.86 and negative-predictive value r = 0.82. Average sensitivity, the most precise measure of screening-test reliability, was 0.87 +/- 0.14 and average specificity 0.52 +/- 0.26.
  
  
  CONCLUSIONS
  Much of the heterogeneity in performance of f/V(T) can be explained by variation in pretest probability of successful outcome, which may be secondary to spectrum and test-referral bias. The average sensitivity of 0.87 indicates that f/V(T) is a reliable screening test for successful weaning.
authors.affiliation: Division of Pulmonary and Critical Care Medicine, Edward Hines Jr Veterans Affairs Hospital, and Stritch School of Medicine, Loyola University of Chicago, Hines, IL 60141, USA. mtobin2@lumc.edu
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Tobin
authors.forename: Amal
authors.initials: A
authors.lastname: Jubran
mesh_terms: D001499:Bayes Theorem; D003657:Decision Making; D006801:Humans; D007362:Intensive Care Units; D011237:Predictive Value of Tests; D013990:Tidal Volume; D015300:Ventilator Weaning