Interferon gamma for diagnosing tuberculous pleural effusions

Abstract

Background - A study was undertaken to evaluate the diagnostic value of pleural fluid concentrations of interferon gamma (IFN-γ) as a marker of tuberculosis. 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. Results - A total of 66 patients were enrolled and tuberculosis was confirmed in 39 of them. The diagnosis in the non-tuberculous group included malignacy (15), paramalignancy (11), and chronic pleuritis secondary to infective endocarditis (1). The mean (SE) IFN-γ level in the pleural fluid was significantly higher in the tuberculous group than in the non-tuberculous group (1493.3 (131.3) pg/ml versus 80.1 (50.4) pg/ml, p < 0.001). The overlap between the two groups was minimal. At the cut off value of 240 pg/ml the sensitivity was 94.9% (95% CI 86.6 to 100), the specificity was 96.3% (95% CI 89.2 to 100), and the likelihood ratio for a positive test result was 25.6. Conclusions - The pleural fluid concentration of IFN-γ is a good and useful diagnostic marker of tuberculosis presenting as a lymphocytic exudative pleural effusion.