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

BACKGROUND Elimination of tuberculosis (TB) in the United States requires treating not only persons with active disease but also those infected with TB. Achieving this goal requires understanding local TB infection prevalence and identifying subgroups at increased risk for infection and disease. METHODS The study population included all patients tested with an interferon-gamma release assay (IGRA) test at New York City (NYC) public TB clinics from October 1, 2006 to December 31, 2011. Patients who were not a case or contact at testing (general clinic patients) and who had a positive QuantiFERON-Gold In-Tube (QFT-GIT) test result were compared with those with indeterminate or negative results to identify characteristics associated with positive results. New York City TB surveillance data were used to identify clinic patients later diagnosed with active TB disease. RESULTS A total of 69 273 IGRA tests were conducted. Among 20 066 patients tested with QFT-GIT, 16% tested positive, 83% tested negative, and <1% were indeterminate. Of 18 481 general clinic patients, 14% had a positive QFT-GIT result. Nine percent of United States-born patients compared with 19% of foreign-born patients had a positive result. Increasing age and birth in a high-incidence country were associated with a higher likelihood of having a positive result. One patient with a negative QFT-GIT result was identified as a TB case 2 years later. CONCLUSIONS Using QFT-GIT data, the background prevalence of TB infection in NYC was estimated. Patient characteristics associated with a positive QFT-GIT result were consistent with known TB risk factors. Results suggest that IGRAs are reliable tests for TB infection.