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

Latent Tuberculosis Infection (LTBI) is a state of persistent immune response to <i>Mycobacterium tuberculosis</i> 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.