T-Spot. TB outperforms tuberculin skin test in predicting development of active tuberculosis among household contacts

Background and ObjectiveIn Hong Kong, neonatal Bacillus Calmette-Guerin (BCG) vaccination is practiced with 99% coverage. This study was to compare the performance of T-Spot.TB and tuberculin skin test (TST) in predicting tuberculosis (TB) among household contacts. MethodsFrom 1 March 2006 to 31 July 2010, 1049 asymptomatic household contacts of smear-positive patients were simultaneously tested with T-Spot.TB and TST, and then followed for up to 5 years for development of TB. Attending clinicians and subjects were blinded to the results of T-Spot.TB. ResultsT-Spot.TB gave a significantly higher positive rate (32.7% vs 22.1%) and better association with exposure time than TST at the 15mm cut-off. Agreement between T-Spot.TB and TST using cut-offs of 5, 10 and 15mm were relatively poor (kappa 0.25-0.41) irrespective of presence or absence of BCG scar. Only T-Spot.TB positivity was negatively associated with BCG scar. Both T-Spot.TB (incidence rate ratio between test-positive and test-negative subjects, IRR: 8.2) and TST (IRR: 4.1, 6.1 and 2.8, using cut-offs of 5mm, 10mm and 15mm, respectively) helped to predict TB. Using a TST cut-off of 15mm, 56% of future TB cases and 62.5% of bacteriologically confirmed cases were missed. Lowering the TST cut-off to 10mm or 5mm could achieve sensitivity comparable with that of T-Spot.TB, but at the expense of lower specificities, with more positive tests (thus requiring treatment) per case of TB predicted. ConclusionsT-Spot.TB outperformed TST in predicting TB among household contacts in a high-income area with widespread BCG vaccination coverage.