title: Role of whole-blood interferon-gamma release assay in the diagnosis of smear-negative tuberculosis.
abstract: METHODS
  Hong Kong Chest Clinics.
  
  
  OBJECTIVE
  To conduct a prospective study investigating the role of a whole-blood interferon-gamma release assay, QuantiFERON®-TB Gold In-Tube (QFT-GIT), in the diagnosis of smear-negative tuberculosis (TB). The QFT-GIT result was compared with the final confirmed diagnosis after 12 months.
  
  
  RESULTS
  Of 262 smear-negative subjects, 188 had active TB, 167 (88.8%) of whom were QFT-GIT-positive; 74 had inactive/non-TB, 30 (40.5%) of whom were QFT-GIT-negative. The positive (PPV) and negative predictive values for active TB were respectively 79.1% and 58.8%. For this target group with high TB prevalence (71.8%), a positive test increased the chance of active disease by only 7.3%. Despite a positive likelihood ratio (LR) of 1.49, the negative LR was 0.28, making the diagnosis of active TB much less likely after a negative test. Although sensitivity and specificity showed no difference across different age groups, the PPV decreased (P < 0.001) with increasing age, likely reflecting the increased prevalence of competing diagnoses.
  
  
  CONCLUSIONS
  In an area with a high prevalence of latent TB infection, a positive QFT-GIT test does not add much to confirm the diagnosis of smear-negative TB, while a negative test indicates a need for further investigation.
authors.affiliation: Tuberculosis and Chest Service, Centre for Health Protection, Department of Health, Hong Kong, China. eric_leung@dh.gov.hk
authors.forename: E C C
authors.initials: EC
authors.lastname: Leung
authors.forename: C C
authors.initials: CC
authors.lastname: Leung
authors.forename: W W L
authors.initials: WW
authors.lastname: Leung
authors.forename: K M
authors.initials: KM
authors.lastname: Kam
authors.forename: W W
authors.initials: WW
authors.lastname: Yew
authors.forename: S N
authors.initials: SN
authors.lastname: Lee
authors.forename: C M
authors.initials: CM
authors.lastname: Tam
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D002648:Child; D005260:Female; D005500:Follow-Up Studies; D006723:Hong Kong; D006801:Humans; D007371:Interferon-gamma / Q000097:blood*; D016013:Likelihood Functions; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D015995:Prevalence; D011446:Prospective Studies; D012680:Sensitivity and Specificity; D013183:Sputum / Q000382:microbiology*; D013997:Time Factors; D014376:Tuberculosis / Q000175:diagnosis*; D055815:Young Adult
chemical_list: D007371:Interferon-gamma