Utility of QuantiFERON-TB Gold In-Tube assay in adult, pulmonary and extrapulmonary, active tuberculosis diagnosis

• The QuantiFERON-TB Gold In-Tube assay (QTF-GIT) should not be recommended routinely for the diagnosis of tuberculosis, as the positive and negative likelihood ratios are of mediocre value (3.2 and 0.2 respectively).
• The assay may also produce false-negative results in some situations: elderly age, high C-reactive protein, low total lymphocyte count, or when immunosuppressants are being taken.
• However, the QTF-GIT may be used as an element of TB diagnosis in certain situations, especially for lymph node TB (where the QTF-GIT level is higher) and in low incidence areas (to limit the number of patients with latent TB).

SummaryBackgroundTuberculosis remains a public health problem in France and the diagnosis of tuberculosis disease (TB) is sometimes difficult. The aim of this study was to analyse the contribution of the QuantiFERON-TB Gold In-Tube assay (QFT-GIT) to TB diagnosis.MethodsSixty patients hospitalized with TB, for whom a QFT-GIT assay had been performed between June 2008 and June 2011 at the University Hospital of Bondy in the north-east of Paris, were identified retrospectively. Clinical and laboratory data were collected. The sensitivity, specificity, predictive values, and likelihood ratios of the QFT-GIT were all calculated. Furthermore, the characteristics of patients testing positive were compared to those of patients testing negative, as well as the QFT-GIT values according to several different factors.ResultsThe sensitivity of the QFT-GIT was 85% (95% confidence interval (CI) 0.73–0.92) and specificity was 73.3% (95% CI 0.68–0.78). The positive predictive value was 39.5% and the negative predictive value was 97.3%. The positive and negative likelihood ratios were 3.2 and 0.20, respectively. The prevalence of TB in this population was 15% (pre-test probability). After a positive test result, the probability of TB increased to 40% (post-positive probability test); after a negative test result, it decreased to 4.5% (post-negative probability test). The combination of the QFT-GIT test with the tuberculin skin test brought no significant improvement in sensitivity. Factors significantly associated with a negative QFT-GIT result included older age, high C-reactive protein, a low lymphocyte count, and immunosuppressant intake. The test value in quantitative terms was significantly higher in those with lymph node TB than in those with pulmonary TB, and in younger patients (<40 years) than in older patients (>40 years old).ConclusionOn its own, QFT-GIT is an insufficient tool to confirm the diagnosis of TB disease. However, it may form part of an ensemble of tools in combination with clinical, biological, and radiological assessments.