title: Limited added value of T-SPOT.TB blood test in diagnosing active TB: a prospective bayesian analysis.
abstract: OBJECTIVE
  To determine the diagnostic value of a blood interferon-gamma release assay in suspected active tuberculosis (TB).
  
  
  METHODS
  136 subjects with suspected pulmonary TB (pTB) at a single London centre with intermediate TB incidence, were clinically graded into low (<25%), medium, or high (>75%) likelihood of active pTB and then tested by T-SPOT®.TB assay. The diagnosis was confirmed by culture (n = 33), treatment response (n = 13) or a firm alternative diagnosis (n = 90).
  
  
  RESULTS
  Overall, the T-SPOT.TB sensitivity was 74% (95% confidence intervals 60-84%), positive predictive value (PPV) 56% (43-68%), negative predictive value (NPV) 83% (71-90%), positive likelihood ratio (PLR) 1.75 and negative likelihood ratio (NLR) 0.45. Results for high pTB likelihood subjects: PPV 100%, NPV 25% (7-60%), PLR >69, NLR 0.31. Results for intermediate pTB likelihood subjects: PPV 67% (41-85%), NPV 88% (65-96%), PLR 2.39, NLR 0.26. Results for low pTB likelihood subjects: PPV 15% (6-34%), NPV 92% (79-97%), PLR 1.23, NLR 0.80. False negatives occurred in 24% of cases of active tuberculosis (4 smear and culture-positive, 3 smear negative and culture-positive, and 4 culture negative).
  
  
  CONCLUSIONS
  The predictive values and likelihood ratios show the T-SPOT.TB test does not assist in confidently confirming or excluding active TB, regardless of the pre-test probability of disease.
authors.affiliation: Department of Medicine, Royal Free Hospital, London NW3 2QG, UK. aileenboyd@doctors.org.uk
authors.forename: Aileen E
authors.initials: AE
authors.lastname: Boyd
authors.forename: Anthony
authors.initials: A
authors.lastname: Ashcroft
authors.forename: Marc
authors.initials: M
authors.lastname: Lipman
authors.forename: Graham H
authors.initials: GH
authors.lastname: Bothamley
mesh_terms: D001499:Bayes Theorem; D019411:Clinical Laboratory Techniques / Q000379:methods*; D005188:False Negative Reactions; D005260:Female; D006801:Humans; D007118:Immunoassay / Q000379:methods; D007371:Interferon-gamma / Q000378:metabolism; D008131:London; D008297:Male; D011237:Predictive Value of Tests; D011446:Prospective Studies; D012680:Sensitivity and Specificity; D014376:Tuberculosis / Q000175:diagnosis*
chemical_list: D007371:Interferon-gamma