METHODS Patients admitted to King Chulalongkorn Memorial Hospital between April 1997 and January 1998 with a lymphocytic exudative pleural effusion were enrolled into the study. The pleural fluids were examined for cytology, staining for acid fast bacilli, and mycobacterial culture. Pathological examination and mycobacterial culture were performed on each pleural biopsy specimen. The diagnosis of tuberculosis was made when one of the following criteria was met: (1) Mycobacterium tuberculosis was isolated from either the pleural fluid or pleural tissue; (2) granulomas were demonstrated in the pleural tissue which stained positive for acid fast bacilli (AFB); or (3) in the presence of granulomas negative on staining for AFB in pleural tissue there was a response to antituberculous treatment on follow up. All pleural fluid samples were stored at –70°C and the IFN-γ level was measured by immunoassay. Analysis was made using sensitivity, specificity, and likelihood ratio for a positive test result. The best cut off point was determined by the highest likelihood ratio and receiver operating characteristic curve.