Estimated Prevalence of Tuberculosis Infection Among a New York City Clinic Population Using Interferon-gamma Release Assays

To continue moving towards elimination of tuberculosis (TB) in the United States requires not only treating current cases of active disease but also addressing the much larger population of persons with TB infection that serves as a reservoir for future TB cases. National guidelines recommend targeted testing and treatment of high-risk populations for TB infection as one part of a comprehensive approach to controlling TB [1]. Implementing this strategy requires knowing the background prevalence of TB infection and identifying the populations at greatest risk. The 1999–2000 National Health and Nutrition Examination Survey (NHANES) measured the prevalence of TB infection in the general population using the tuberculin skin test (TST) [2]. The NHANES data were collected to be representative on a national level and are not necessarily reflective of local epidemiology; this is especially true for urban and metropolitan areas, such as New York City (NYC), which have large and diverse populations that cannot be characterized by extrapolating national data [3]. A study of patients tested with the TST in NYC Department of Health and Mental Hygiene (DOHMH) TB clinics from 2002 to 2004 found a TB infection prevalence of 24% [4]. Since this study, new blood-based tests for TB infection have become widely available. These interferon-gamma release assays(IGRAs) are thought to be an improvement over the TST because they are more specific to Mycobacterium tuberculosis complex, do not yield false-positive results due to prior bacillus Calmette-Guerin ( BCG) vaccination, and do not cross-react with most nontuberculous mycobacteria[5–7]. In addition, they require only a single medical visit and produce instrument-derived quantitative results, which are not subject to reader bias [8–10]. New York City is the city in the United States with the highest TB burden, with 651 cases reported in 2012 [11, 12]. The NYC DOHMH operates 4 TB clinics that provide TB evaluation and treatment services free of charge to the patient. These clinics handle over 50 000 patient visits and conduct over 5000 TB infection tests annually [12]. New York City began using the QuantiFERON-Gold IGRA (Cellestis Limited, Carnegie Victoria, Australia) as the standard diagnostic test for TB infection in DOHMH TB clinics in 2006; in November 2009, the clinics began using the next generation QuantiFERON-Gold In-Tube test (Cellestis Limited). Data from this setting offer a unique opportunity to establish estimates of TB prevalence and identify higher-risk groups within this patient population [4]. We present an analysis of IGRA tests conducted in NYC TB clinics, including the characteristics associated with having a positive test result.