The UK PREDICT tuberculosis cohort study compared the prognostic value of two interferon-γ release assays (IGRAs) and a tuberculin skin test (TST) in predicting the development of active tuberculosis among high-risk individuals (50.6% contacts and 49.4% migrants) who were not undergoing preventive treatment.
Participants were tested for latent tuberculosis infection by use of three screening procedures - T-SPOT.TB, QuantiFERON-TB Gold In-Tube, and TST. For TST, there were three thresholds for a positive result dependent on the size of skin induration: TST-5 at 5 mm, TST-10 at 10 mm, and TST-15, which was a BCG vaccination status dependent measure, at 5 mm for BCG-naive and at 15 mm for BCG-vaccinated participants.
T-SPOT.TB, QuantiFERON-TB Gold In-Tube, and the BCG-stratified TST-15 were significantly better predictors of progression than TST-5 or TST-10. Although TST-5 identified more participants who progressed to active tuberculosis than TST-10 or TST-15, it also classified large groups of individuals as infected who then did not progress.
The authors conclude that their findings provide evidence that TST stratified by BCG-vaccination status (TST-15) yields similar predictive values to the two commonly used IGRAs, supporting their use in screening programmes.
A related Comment notes that “the key issue in practice is to select among the individuals who are latently infected with tuberculosis in these two populations those who have the highest risk of disease progression and who will benefit most from a preventive treatment. This selection should be on the basis of not only the result of a blood or skin test, but also individual factors like age, intensity of exposure, and general state of health and immunological defences. Combining these data could help decrease the number of people who need to be treated to prevent a single future case of tuberculosis, thus reducing the unnecessary prescription of drugs.”