Mycobacterium tuberculosis (MTB)-specific interferon-γ release assays (IGRAs) are an alternative or adjunct to the tuberculin skin test (TST) in identifying recent contacts with latent tuberculosis infection (LTBI), but there are scarce data directly comparing performance of the tests.
To evaluate the agreement between both IGRAs and to determine which contacts were most likely to represent LTBI, the QuantiFERON-TB-Gold In Tube assay (QFT) and the T-Spot.TB test (T-Spot) were compared in TST-positive persons recently exposed to pulmonary tuberculosis cases.
Prospectively enrolled close contacts (n = 812) of 123 culture-confirmed tuberculosis source cases underwent IGRA testing using standardized collected data. Factors independently influencing the risk of MTB infection and their interactions with each other were evaluated by multivariate analysis.
Five variables were found to significantly predict a positive IGRA test result (age, source case acid-fast bacilli positive and/or coughing, cumulative exposure time, foreign origin). There was excellent agreement between the two IGRAs (93.9%, κ = 0.85), with QFT finding 30.2% of contacts positive and T-Spot finding 28.7%. Assuming positivity to both IGRAs as true infection, sensitivity of the TST at > 10 mm was 72% and at > 15 mm was 39.7%. The use of either IGRA as a replacement for the TST would decrease the number of LTBI suspects to be investigated by approximately 70%.
IGRAs are a more accurate indicator of the presence of LTBI than the TST. Both QFT and T-Spot appear to be valuable public health tools, showing excellent agreement with each other.