High LTBI positivity rates due to methotrexate. False Positives

Introduction: Patients with Rheumathoid Arthritis (RA) have far higher percentages of positive Tuberculin Skin Test (TST) results compared with the Interferon Gamma Release assays (IGRA). Objetive: To determine the influence of different treatments used in patients with rheumatic diseases on the performance of LTBI screening tests. Material and Methods: LTBI diagnosis was based on a history of contact with an active TB, chest X Ray indicative of LTBI, a positive TST, and/or a positive IGRA. Determination of T-cell subpopulations was also performed. To verify the results peripheral blood mononuclear cells were isolated. Cultures were performed in RPMI 1640 medium. Cells were incubated at 37°C under 5% carbon dioxide. Results: Between April 29, 2013 and March 29, 2016, 393 patients were prospectively included.TST was positive for 22.2% ankylosing spondylitis (SpA) patients, for 25% rheumatoid arthritis (RA) patients and 35.7% of Psoriatic Arthritis (PA) patients. QFT positive results were 15.6% (SpA), 20% (RA), 19% (PA). T.SPOT TB positive results were: 13.5% (SpA), 14, 2% (RA), 15.3% (PA). Figure 1 shows the association between MTX dose and the risk of a positive result for TST. Main findings: 1.MTX appears to be related to a high number of TST positive results in patients with rheumatic diseases. 2.T-lymphocytes from MTX treated patients are higher IFN- γ producers at low doses of anti-CD3 than from MTX untreated patients.