Determining Risk in Latent Tuberculosis

The efficacy of treating tuberculin skin test (TST) positive, or interferon gamma release assay (IGRA) positive, contacts of tuberculosis (TB) cases to prevent progression to disease is well established. However the length of treatment, and the toxicity associated with the currently used regimens, means that the risk may outweigh the benefit and treatment completion rates are poor. In addition, no proven regimens are available for contacts of multidrug resistant tuberculosis (MDR-TB) cases. Because as few as 2% of contacts develop active TB over 1 year and no surrogate markers are available, drug trials to assess novel treatments typically require thousands of subjects followed up for many years. (18F)-fluoro-2-deoxy-D-glucose positron emission tomography/computer tomography (FDG-PET/CT) may prove a useful surrogate for more targeted chemoprophylaxis as well as a means to rapidly evaluate novel prophylactic regimes in future studies. Up to 40% of immune-sensitized TB contacts with normal chest radiographs (CXR) have abnormalities on conventional chest CT. FDG-PET/CT not only will allow characterization of the metabolic activity of these lesions but is also likely to reveal significantly increased metabolic activity within regional lymph nodes that may otherwise be anatomically normal. Based on previous studies, we predict that up to 65% of contacts will have combined chest PET/CT abnormalities and that up to 50% of contacts who are treated, will have increased FDG uptake that will resolve with treatment. By contrast, PET screening studies demonstrate abnormal pulmonary FDG uptake occurs in 0.9% of healthy individuals. The development of biomarkers more predictive of disease progression is also highly desirable, but for similar reasons evaluating them is challenging. This novel approach of using FDG-PET/CT to benchmark the dynamic immunological, transcriptional, or metabolic changes that occur early in tuberculosis infection, we hope will accelerate biomarker discovery. In this study we propose to evaluate these predictions in order to lay the foundation for future studies.