Serial testing with the interferon-γ release assay in Portuguese healthcare workers

Evidence for the utility of the new Mycobacterium tuberculosis (MTB) specific IFN-gamma release assays in diagnosing latent tuberculosis infection (LTBI) is growing. However, data concerning conversion and reversion rates in serial testing of healthcare workers (HCWs) with an interferon-gamma release assay are sparse. Between February 2007 and September 2009, 670 HCWs in the University Hospital of Porto, Portugal were tested at least twice with QuantiFERON-TBA (R) Gold In-Tube (QFT) for LTBI. The tuberculin skin test (TST) was performed simultaneously. QFT was considered positive if INF-gamma A a parts per thousand yen0.35 IU/mL. TST conversion was defined as an increase a parts per thousand yen10 or a parts per thousand yen6 mm compared to a baseline TST < 10 mm. The second QFT was positive in 4.8% of the 376 HCWs with an INF-gamma concentration at baseline below 0.1 IU/mL but in 48.8% of the 41 HCWs with an INF-gamma concentration of 0.2 to < 0.35 IU/mL. Out of 74 HCWs with a baseline INF-gamma concentration a parts per thousand yen3.0 IU/mL, 4 (5.4%) reversed while 27 out of 55 HCWs (49%) with a baseline INF-gamma concentration a parts per thousand yen0.35 to < 0.7 IU/mL reversed to a negative QFT. Those 61 HCWs with TST conversion (increase a parts per thousand yen10 mm) were most often (78.7%) negative in both consecutive QFTs. Our data suggests the use of an uncertainty zone between 0.2 and 0.7 IU/mL in serial testing with QFT. As long as the knowledge regarding disease progression in QFT-positive persons is limited, persons pertaining to this zone should be retested before being offered preventive chemotherapy.