Abstract
Background
Tuberculosis (TB) is an important public health issue, and new entrant screening is vital to identify and treat latent TB infection (LTBI) and reduce progression to active disease. Single-step interferon-gamma release assay (IGRA) testing is cost-effective and recommended by NICE, but the positive predictive value (PPV) of QuantiFERON® (QFT) for progression to active TB in new entrant screening has not been previously reported. We aimed to study this using our experience in these screening programmes.
Method
Using local databases, we identified all new entrants screened with QFT in Leeds and Blackburn between 2007 and 2012, and which of these received chemoprophylaxis. Progression to active TB was identified from the national Enhanced Tuberculosis Surveillance database, and TB-free survival was compared between those with untreated LTBI, and those with negative QFT.
Results
Of 4511 patients screened, QFT was positive in 1092 (24%). Of these, 21 were diagnosed with active TB at screening, and 1071 with LTBI. Of those with LTBI, 600 (56%) received chemoprophylaxis, 2 (0.33%) of whom progressed to active TB. Of the 471 not treated, 15 (3.18%) progressed to active TB, compared with 4 (0.01%) of those with negative QFT.
The PPV of a positive QFT for progression to active TB within 5 years was calculated at 21.8% (95% CI 12.7-33.3%). The derived protective efficacy of chemoprophylaxis was 89%.
Conclusion
The PPV of a positive QFT for progression to active TB in this population was 21.8%, which is relevant in decision-making around LTBI treatment. This is higher than previous estimates, and improves the cost effectiveness of screening.
- © 2014 ERS