Indeterminate test results of T-SPOT.TB performed under routine field conditions.

Interferon-gamma release assays for the diagnosis of tuberculosis (TB) can give indeterminate results. The prevalence of indeterminate test results (ITRs) among T-SPOT.TB tests was assessed. A retrospective analysis of samples processed in 2005 was performed. ITRs were assessed by age, sex, immunosuppression, distance to the laboratory and season. A subgroup of tests performed for specific indications (contact tracing, migrants with positive tuberculin skin test, TB suspects and immunosuppression) were analysed separately. Of a total of 1,429 tests, 49 (3.4%) were indeterminate. ITRs were significantly associated with old age (>75 versus 5-75 yrs; odds ratio (OR) 7.97, 95% confidence interval (CI) 3.968-15.438) and the season during which samples were transported (autumn and winter versus spring and summer; OR 3.47, 95% CI 1.753-7.514). The incidence of ITR was 302 (2.0%) among TB contacts, 75 (1.6%) among immigrants, 156 (3.0%) in TB suspects and 32 (3.0%) among immunosuppressed patients. Sex, young age and distance to the laboratory were not associated with the rate of ITR. Of the 13 tests with ITR that were repeated, 10 gave a clear positive or negative result. Indeterminate test results with T-SPOT.TB under routine conditions were infrequent and more common in individuals aged >75 yrs than in children and younger adults. The incidence of indeterminate test results was low and similar among healthy tuberculosis contacts, immigrants with a positive tuberculin skin test, tuberculosis suspects and the immunosuppressed. The conditions of transportation may influence the incidence of indeterminate test results.

Contact tracing using a new T-cell-based test: better correlation with tuberculosis exposure than the tuberculin skin test.

SETTING: Residential institution for alcoholics in Switzerland. OBJECTIVE: To compare the results of the tuberculin skin test (TST) and the new T-cell-based test for tuberculosis infection (T-SPOT.TB) in subjects exposed to a case of smear-positive pulmonary TB (PTB). DESIGN: After the notification of smear-positive PTB in a resident of an institution for alcoholics, contacts underwent TST and determination of Mycobacterium tuberculosis specific T-cells in blood by T-SPOT.TB. Results were analysed according to age, history of BCG vaccination, and level of exposure to the index case. RESULTS: There was no correlation between the level of exposure and the TST results, but the T-SPOT.TB results were significantly correlated with the level of exposure (P = 0.029, OR 5.00, 95%CI 1.05-23.86). Contacts who had been previously BCG-vaccinated were significantly more likely to have a positive TST than unvaccinated contacts (52% vs. 0%, P = 0.0003), but there was no influence of prior BCG vaccination on T-SPOT.TB results. CONCLUSIONS: T-SPOT.TB test results correlated better than TST with level of exposure to M. tuberculosis and were not confounded by prior BCG vaccination. This test allows better selection of contacts who should receive treatment for latent TB infection.