[Interferon-gamma release assays in tuberculosis diagnostics]

Evaluation of cell-mediated immune responses to two BCG vaccination regimes in young children in South Korea

Children in South Korea are vaccinated with either BCG Pasteur vaccine intradermally (ID), or with BCG Tokyo vaccine given by multipuncture device (MP). Data from a recent national survey indicated that in children under 6 years old, 31.1% had received the ID vaccine and 64.5% the MP vaccine. To compare the T cell responses induced by the two vaccines, children aged 3-7 were recruited and tested for tuberculin skin test reactivity and for in vitro IFN-γ responses to mycobacterial antigens. DTH responses were not significantly different in children vaccinated by either the ID or MP vaccines. PPD-induced IFN-γ was measured in supernatants of 6-day diluted whole blood cultures. IFN-γ production to PPD was not significantly different in the two vaccine groups, although there is a trend that the MP group gives a higher proportion of IFN-γ positivity than the ID group. In addition, when IFN-γ responses to the antigens ESAT-6 and CFP-10 were assessed in the 6-7 year old group, there was no significant difference between the two vaccine groups. Thus, there was no evidence that the increasing use of MP vaccination has reduced protection against M. tuberculosis in young children in South Korea, based on immunogenicity as assessed by DTH and IFN-γ responses to PPD, and also equivalent frequency of responses to ESAT-6 and CFP-10.

Performance of the tuberculin skin test and interferon-gamma release assay for detection of tuberculosis infection in immunocompromised patients in a BCG-vaccinated population

BACKGROUND

Interferon-gamma release assay (IGRA) may improve diagnostic accuracy for latent tuberculosis infection (LTBI). This study compared the performance of the tuberculin skin test (TST) with that of IGRA for the diagnosis of LTBI in immunocompromised patients in an intermediate TB burden country where BCG vaccination is mandatory.

METHODS

We conducted a retrospective observational study of patients given the TST and an IGRA, the QuantiFERON-TB Gold In-Tube (QFT-IT), at Severance Hospital, a tertiary hospital in South Korea, from December 2006 to May 2009.

RESULTS

Of 211 patients who underwent TST and QFT-IT testing, 117 (55%) were classified as immunocompromised. Significantly fewer immunocompromised than immunocompetent patients had positive TST results (10.3% vs. 27.7%, p 0.001), whereas the percentage of positive QFT-IT results was comparable for both groups (21.4% vs. 25.5%). However, indeterminate QFT-IT results were more frequent in immunocompromised than immunocompetent patients (21.4% vs. 9.6%, p 0.021). Agreement between the TST and QFT-IT was fair for the immunocompromised group (kappa = 0.38), but moderate agreement was observed for the immunocompetent group (kappa = 0.57). Indeterminate QFT-IT results were associated with anaemia, lymphocytopenia, hypoproteinemia, and hypoalbuminemia.

CONCLUSION

In immunocompromised patients, the QFT-IT may be more sensitive than the TST for detection of LTBI, but it resulted in a considerable proportion of indeterminate results. Therefore, both tests may maximise the efficacy of screening for LTBI in immunocompromised patients.

Comparison of Screening Procedures for Mycobacterium tuberculosis Infection Among Patients with Inflammatory Diseases

Objective.

To test if Mycobacterium tuberculosis screening results differ among patients with inflammatory disease depending on whether the QuantiFeron TB-Gold test (QFT) or tuberculin skin test (TST) is used; and to evaluate if a possible difference is influenced by the presence of risk factors or immunosuppression.

Methods.

The interferon-γ response to in vitro stimulation of M. tuberculosis-specific antigens was measured with QFT and results were compared with TST. Associations to bacillus Calmette-Guerin (BCG) vaccination, risk factors, and immunosuppression were analyzed for both tests.

Results.

QFT and TST results were available for 294/302 and 241/302 patients, respectively; 234 had results from both tests. Twenty-one (7%) tested positive with QFT and 45 (19%) with TST. A positive QFT was associated with risk factors for M. tuberculosis infection: i.e., birth or upbringing in a TB-endemic area [risk ratio (RR) = 7.8, 95% CI 1.5–18.2, p < 0.001], previous TB treatment (RR 4.7, 95% CI 1.6–13.5, p = 0.005), and any latent TB infection risk factor (RR 4.7, 95% CI 2.1–11.0, p = 0.0002). Treatment with corticosteroids increased the risk for an inconclusive QFT result (RR 4.2, 95% CI 1.6–10.7, p = 0.04) and decreased the risk for a positive TST result (RR 0.4, 95% CI 0.1–1.0, p = 0.04). Agreement between the tests was low (kappa 0.2, 95% CI 0.02–0.3, p = 0.002).

Conclusion.

The study documented a high degree of discordant positive QFT and TST results. A positive QFT was more closely associated with risk factors for M. tuberculosis infection than the TST. The use of corticosteroids affected test outcome by increasing the risk for an inconclusive QFT result and decreasing the risk for a positive TST result.