Interferon gamma for diagnosing tuberculous pleural effusions

Background-A study was undertaken to evaluate the diagnostic value of pleur al fluid concentrations of interferon gamma (IFN-gamma) as a marker of tube rculosis. Methods-Patients admitted to King Chulalongkorn Memorial Hospital between A pril 1997 and January 1998 with a lymphocytic exudative pleural effusion we re enrolled into the study. The pleural fluids were examined for cytology, staining for acid fast bacilli, and mycobacterial culture. Pathological exa mination and mycobacterial culture were performed on each pleural biopsy sp ecimen. The diagnosis of tuberculosis was made when one of the following cr iteria was met: (1) Mycobacterium tuberculosis was isolated from either the pleural fluid or pleural tissue; (2) granulomas were demonstrated in the p leural 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 degrees C and the IFN-gamma level was mea sured by immunoassay. Analysis was made using sensitivity, specificity, and likelihood ratio for a positive test result. The best cut off point was de termined by the highest likelihood ratio and receiver operating characteris tic curve. Results-A total of 66 patients were enrolled and tuberculosis was confirmed in 39 of them. The diagnoses in the nontuberculous group included malignan cy (15), paramalignancy (11), and chronic pleuritis secondary to infective endocarditis (1). The mean (SE) IFN-gamma level in the pleural fluid was si gnificantly higher in the tuberculous group than in the nontuberculous grou p (1493.3 (131.3)pg/ml versus 80.1 (50.4)pg/ml, p < 0.001). The overlap bet ween the two groups was minimal. At the cut off value of 240 pg/ml the sens itivity was 94.9% (95% CI 86.6 to 100), the specificity was 96.3% (95% CI 8 9.2 to 100), and the likelihood ratio for a positive test result was 25.6. Conclusions-The pleural fluid concentration of IFN-gamma is a good and usef ul diagnostic marker of tuberculosis presenting as a lymphocytic exudative pleural effusion.