Screening for latent tuberculosis before starting TNF-alpha inhibitors in a population with high BCG vaccination rates

It is assumed that in candidates for TNF-alpha inhibitor (TNFi) treatment, tuberculin skin test (TST) may be unreliable, since BCG vaccination causes false positive and drugs cause false negative results, favoring the use of Quantiferon or T-spot assays. However, these tests may not be readily available in all parts of the world. We aimed to determine the reliability of TST with respect to BCG vaccination and drugs in candidates for TNFi treatment, and how isoniazid is tolerated, assuming that the use of TST would result in increased isoniazid use. We included 1031 adult patients who were prescribed a TNFi for the first time. We analysed the association of BCG and drugs with TST and Quantiferon results, the determinants of a positive TST, and evaluated the tolerability of isoniazid. BCG vaccination and male sex were associated with positive TST (OR 3.56, 95% CI 1.98–6.41 and OR 2.54, 95% CI 1.75–3.68, respectively), while prednisolone and azathioprine were associated with negative TST (OR 0.63, 95% CI 0.43–0.91 and OR 0.40, 95% CI 0.11–0.76). Isoniazid was prescribed to 684 (66.3%) patients and had to be discontinued in 12.2% of these before 9 months, most commonly due to hepatotoxicity (44%). One patient developed tuberculosis despite isoniazid use. BCG vaccination may be associated with false positive TST, despite a long time since vaccination in candidates for TNFi treatment. Prednisolone and azathioprine use were associated with negative TST. Despite the high frequency of isoniazid use associated with using TST instead of QTF, isoniazid was generally well tolerated.