Diagnosis of extrapulmonary tuberculosis (TB) is often challenging. In this work we discuss the utility of an assay for Lipoarabinomannan (LAM) antibody detection in synovial fluid. LAM is one of the three major groups of lipopolysaccharides within the Mycobacterium tuberculosis (MTB) cell wall. An ELISA based test was used to investigate the presence of antibodies against LAM in an immunocompetent patient with knee arthritis. The symptoms resolved after isoniazid treatment. LAM positivity has been used as a diagnostic tool for TB in different settings, including veterinary field. The test could be of some value to diagnose tuberculous arthritis in selected patients when gold standard test returned negative although further investigations are welcome.