Introduction TNF-α plays an important role in host defense against various infections and in the pathogenesis of chronic inflammatory diseases. Therapeutic blockade of TNF-α could be an effective treatment in such cases. However, anti-TNF-α therapy increases the risk of reactivation of serious infections, including tuberculosis (TB). Thus, screening for TB became mandatory, prior to the initiation of TNF-α inhibitor therapy.
Aim of the study To assess the performance and characteristics of the Quantiferon TB Gold test (QFT) by screening for TB in three different populations in Hungary: patients with inflammatory rheumatic diseases (RD), health-care workers who serve homeless people (HW) and homeless clients (HL).
Methods Between April 2011 and March 2014, samples of 1430 consecutive RD patients, who required or already underwent TNF-a blocking therapy, were screened for latent TB. Two untreated groups, (i) 120 HL and (ii) 46 HW, both considered to be at high risk for TB, were also tested. The QFT was performed according to the manufacturer instructions. The amount of released IFN-g was measured by ELISA in samples stimulated by TB Antigen, saline (NIL) and phytohemagglutinin A (PHA).
Results In the RD group, the QFT was negative in 87%, positive in 11%, and indeterminate in 2%. Of the 265 patients, 36 (13.6%) showed inconsistent results on repeat testing. In the HW group, the QFT was negative in 63% and positive in 37%. In the HL group, the QFT was negative in 50.8%, positive in 46.7% and indeterminate in 2.5%. In the RD group, low mitogen response (caused mainly by low lymphocyte count and/or improper specimen handling) and positive results close to the cut-off value of 0.35 IU/mL account for 86% of inconsistent results, while true reversions or conversions only for 14%.
Conclusions Based on our results, for better differentiation of non-specific variations, we suggest that a grey zone close to the cut-off value of 0.35 IU/mL (0.35–0.800 IU/mL) should be defined. As for the mitogen response, results higher than 0.5 IU/mL (cut-off), but lower than 2 IU/mL should be interpreted carefully. Repeat testing in such cases could prevent misinterpretations caused by technical errors.
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