Performance of the T-SPOT.TB test in patients with indeterminate QuantiFERON-TB Gold Plus results: proposal for an algorithm for the diagnosis of Latent Tuberculosis Infection

complex antigens without clinical, radiological and microbiological signs of active disease. Effective diagnosis and preventive treatment of LTBI are crucial for tuberculosis (TB) control, especially in high-risk groups. Currently, two main tests are used for LTBI diagnosis: the Tuberculin Skin Test (TST) and the Interferon-Gamma Release Assays (IGRA), including the QuantiFERON-TB Gold Plus (QFT-Plus) and the T-SPOT.TB. Our study evaluated the performance of the T-SPOT.TB test in patients with indeterminate QFT-Plus results, using data from the Clinical Microbiology and Virology Laboratory (M&V) of Papa Giovanni XXIII Hospital in Bergamo, Italy. Blood samples from patients tested for LTBI with QFT-Plus from January 1, 2017 to May 15, 2024 were analyzed. The QFT-Plus is the most widely used test in routine diagnostics for LTBI screening due to the availability of automated systems. Out of 20,995 samples tested with QFT-Plus, 576 (2.7%) gave indeterminate results. In all cases of indeterminate QFT-Plus results, M&V recommends performing the T-SPOT.TB test. However, of the 576 patients who obtained an indeterminate outcome, only 137 (23.8%) followed the indication. The T-SPOT.TB provided a definitive result in 87.6% of the cases, resolving 120 (80 negative and 40 positive) of 137 indeterminate QFT-Plus outcomes. Specifically, 78 of 92 cases, equal to 84.8%, were settled when the T-SPOT. TB test was performed within 30 days of the QFT-Plus. The T-SPOT.TB test has shown potential effectiveness in addressing indeterminate QFT-Plus results (84.8% resolution), indicating its possible role as a complementary diagnostic tool for LTBI. The proposed algorithm for LTBI screening is based on national and international guidelines recommending the use of the TST and/or an IGRA test for individuals at risk. However, it particularly emphasizes the use of QFT-Plus, due to its practicality and rapid execution, while recommending the addition of the T-SPOT.TB within 30 days in cases of indeterminate QFT-Plus results. Nevertheless, the conclusions should be regarded as preliminary and require confirmation through larger or controlled studies.