Discordance in latent tuberculosis (TB) tests results in end stage renal disease patients

OBJECTIVES: This natural experiment was designed to assess the impact of exposure to an active case of tuberculosis in a group of immunosuppressed individual, with end stage renal disease over an extended follow up. STUDY DESIGN: Close contacts of people with sputum smear positive Mycobacterium tuberculosis are at high risk of infection, particularly immunosuppressed individuals. An infectious tuberculosis health care worker worked in a renal dialysis unit for a month before diagnosis, with 104 renal dialysis patients exposed for ≥8 hours. METHODS: Patients were informed and invited for screening 8-10 weeks post-exposure. They either underwent standard two-step assessment with tuberculin skin test (TST) and QuantiFERON®-TB Gold [Cellestis GmbH] (QFN) interferon gamma release assay (IGRA); or following consent, enrolled in a study where these two tests were performed simultaneously with T-SPOT®-TB [Oxford Immunotec Ltd] (TSPOT). Patients within the study were followed up for two years from exposure;with QFN and TSPOT repeated at months 3 and 6 from first testing RESULTS: Of 104 exposed individuals, 75 enrolled in the study. There was a high degree of discordance between QFN, TSPOT and TST. This was seen at both the first timepoint, and also over time in subjects who were re-tested. No patients had active TB at baseline testing. None received treatment for latent TB infection. Over the following two years, no-one developed TB disease. CONCLUSION: This study suggests there is a low risk of progression to active TB in low incidence countries even in high risk groups. This plus the degree of test result discordance emphasises the complexities of managing TB in such settings, as it is unclear which of these tests, if any, provides the best diagnostic accuracy.