To the Editors: In patients latently infected with Mycobacterium tuberculosis , immunosuppression significantly augments the risk of progression to active tuberculosis (TB) and TB is still one of the most frequent opportunistic infections worldwide. Prevention of TB in HIV-positive patients using the tuberculin skin testing (TST) followed by targeted preventive treatment is an effective strategy, but has thus far shown limited clinical success, in part due to the lack of a reliable test for latent TB infection (LTBI) 1. Interferon (IFN)-γ release assays (IGRAs) have shown great potential in the improvement of LTBI diagnosis, but the tests perform suboptimally in immunocompromised populations. We and others have previously shown that HIV-positive patients have high rates of indeterminate QuantiFERON®-TB Gold In-Tube test (QFT-IT) results and that this, at least in part, is due to low CD4 T-cell count 2–4. We and others have also shown that IFN-γ-inducible protein 10 (IP-10/CXCL-10) is an alternative biomarker may improve immunodiagnosis of M. tuberculosis infection. IP-10 is induced specifically and in large quantities upon in vitro stimulation of whole blood with M. tuberculosis -specific antigens. We have developed an IP-10 based test 5 and shown that the IP-10 test performs comparably to the QFT-IT in adult patients with intact immune function 6. We compared the sensitivity of an IP-10 based test with the QFT-IT for diagnosing M. tuberculosis infection in HIV-negative and HIV-positive patients with culture-confirmed active pulmonary tuberculosis (PTB). 300 patients newly diagnosed with PTB were recruited prospectively through the National Tuberculosis and Leprosy Programme (Mwanza, Tanzania). Blood was drawn for IGRA and HIV testing, and CD4 cell counts. Sputum was collected for microscopy and M. tuberculosis culture. Patients >15 yrs of age with a positive sputum culture result were enrolled. All HIV-positive patients were newly diagnosed in the study …