Interferon γ assay for detecting latent tuberculosis infection in rheumatoid arthritis patients during infliximab administration

In rheumatoid arthritis ( RA) patients treated with infliximab ( IFX), QuantiFERON-TB Gold ( QFT-G), an interferon gamma assay for diagnosing tuberculosis infection, was performed to compare its effectiveness to conventional diagnostic procedures ( tuberculin skin test, imaging and medical history) in diagnosing latent tuberculosis infection ( LTBI). QFT-G was measured bimonthly in 14 rheumatoid arthritis patients during IFX treatment. Seven of 14 patients were confirmed as LTBI positive by at least one method. Of these, four were positive on QFT-G during the study period, and two were positive before the start of IFX administration. For two of the four QFT-G-positive patients, LTBI was diagnosed only by QFT-G. The rate of agreement between QFT-G and conventional procedures was 64.3%. A total of 5% of QFT-G tests were impossible to judge due to decreased reactions in the positive control. These results suggest that QFT-G is able to detect LTBI in RA patients overlooked by conventional methods. Conventional procedures and QFT-G should be employed in parallel, and LTBI should be assumed when one technique gives a positive result.