ABSTRACT
TB arthritis is a very rare extrapulmonary presentation in an immunocompetent host. It is usually the result of direct hematogenous spread from the primary focus. Our patient presented with pain and swelling of the right knee for 6 months. The blood investigations and CT chest revealed findings consistent with active tuberculosis. Synovial fluid was positive for acid-fast bacilli (AFB) which is a very rare finding. Cartridge-based nucleic acid amplification test (CBNAAT) revealed Mycobacterium tuberculosis and sensitivity to rifampicin. Establishing the diagnosis of Mycobacterium tuberculosis beyond doubt is very important, and early initiation of antitubercular treatment (ATT) is important as delay in treatment may lead to irreversible damage to the joint and restriction of joint mobility.