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

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Abstract

Background and Objective

In Hong Kong, neonatal Bacillus Calmette–Guerin (BCG) vaccination is practiced with 99% coverage. This study was to compare the performance of T-Spot.TBand tuberculin skin test (TST) in predicting tuberculosis (TB) among household contacts.

Methods

From 1 March 2006 to 31 July 2010, 1049 asymptomatic household contacts of smear-positive patients were simultaneously tested with T-Spot.TBand 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.

Results

T-Spot.TBgave a significantly higher positive rate (32.7% vs 22.1%) and better association with exposure time than TST at the 15 mm cut-off. Agreement between T-Spot.TBand TST using cut-offs of 5, 10 and 15 mm were relatively poor (kappa 0.25–0.41) irrespective of presence or absence of BCG scar. Only T-Spot.TBpositivity 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 5 mm, 10 mm and 15 mm, respectively) helped to predict TB. Using a TST cut-off of 15 mm, 56% of future TB cases and 62.5% of bacteriologically confirmed cases were missed. Lowering the TST cut-off to 10 mm or 5 mm 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.

Conclusions

T-Spot.TBoutperformed TST in predicting TB among household contacts in a high-income area with widespread BCG vaccination coverage.

SUMMARY AT A GLANCE

In a high-income area with high BCG vaccination coverage and low risk of ongoing transmission, T-Spot.TB outperforms TST in predicting TB among household contacts, irrespective of the cut-off used for the latter test.

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