Evaluation of cut-off values of interferon-gamma-based assays in the diagnosis of M. tuberculosis infection

Int J Tuberc Lung Dis. 2008 Jan;12(1):50-6.


Setting: A chest disease centre in Istanbul, Turkey.

Objective: The diagnostic accuracy of interferon-gamma-based assays for Mycobacterium tuberculosis infection may be improved by using lower cut-off values for the tuberculin skin testing (TST), QuantiFERON-TB Gold (QFT) and T-SPOT.TB (T-SPOT) assays.

Design: Three assays, TST, QFT and T-SPOT, were evaluated for their diagnostic performance with respect to different cut-off values. This evaluation was carried out in a comparative study involving 100 patients with untreated culture-confirmed cavitary pulmonary tuberculosis (TB) and 47 healthy subjects.

Results: The sensitivities of the assays were: TST 70%, QFT 78% and T-SPOT 83.5%, while their specificities were TST 35%, QFT 89.4% and T-SPOT 84.8%. Both QFT and T-SPOT were significantly more specific than TST (both P < 0.001), but were similiar to each other (P = 0.5). Receiver operating characteristic analysis revealed that a cut-off value of 0.818 IU/ml for QFT maximises specificity without significant loss of test sensitivity. Using lower cut-off values for T-SPOT and TST, however, also increased the sensitivity of the assay but resulted in a significant decrease in specificity.

Conclusion: Lower cut-off values for TST, QFT and T-SPOT increased the sensitivity of each assay, but only with a lower cut-off value for QFT could specificity be maintained.

Publication types

  • Comparative Study
  • Evaluation Study

MeSH terms

  • Adult
  • Antigens, Bacterial / immunology*
  • Cells, Cultured
  • Female
  • Humans
  • Interferon-gamma / blood*
  • Male
  • Middle Aged
  • Mycobacterium tuberculosis / immunology*
  • Predictive Value of Tests
  • ROC Curve
  • Reproducibility of Results
  • Sensitivity and Specificity
  • Tuberculin / immunology*
  • Tuberculin Test*
  • Tuberculosis, Pulmonary / diagnosis*
  • Tuberculosis, Pulmonary / microbiology


  • Antigens, Bacterial
  • Tuberculin
  • Interferon-gamma