BACKGROUND--Pleural biopsy is usually considered important for the diagnosis of pleural effusions, especially for distinguishing between tuberculosis and neoplasia, even though tuberculous pleural fluid contains sensitive biochemical markers. In regions with a high prevalence of tuberculosis, and in patient groups with a low risk of other causes of pleurisy, the positive predictive value of these markers is increased. The criteria for performing a pleural biopsy under these circumstances have been investigated, using adenosine deaminase (ADA) as a pleural fluid marker for tuberculosis. METHODS--One hundred and twenty nine patients with a pleural effusion aged < or = 35 years (mean (SD) 25.2 (4.9) years) were studied. Seventy three were men. Eighty one effusions (62.8%) were tuberculous, 12 (9.3%) parapneumonic, and 10 (7.7%) neoplastic, five were caused by pulmonary thromboembolism, four by systemic lupus erythematosus, seven by empyema, three following surgery, one was the result of asbestosis, and one of nephrotic syndrome. In five cases no definitive diagnosis was reached. ADA levels were determined by the method of Galanti and Giusti. RESULTS--The diagnostic yield of procedures not involving biopsy was 94.5% (122/129). Pleural biopsy provided a diagnosis in a further two cases, but not in the remaining five. All tuberculous cases had pleural fluid levels of ADA of > 47 U/l (mean (SD) 111.1 (36.6) U/l). The only other cases in which ADA exceeded this level were six of the seven patients with empyema. Cytological examination of the pleural fluid diagnosed eight of the 10 neoplastic cases, compared with six diagnosed by pleural biopsy. CONCLUSIONS--In a region with a high prevalence of tuberculosis procedures not involving pleural biopsy have a very high diagnostic yield in patients with a pleural effusion aged < or = 35 years, making biopsy necessary only in cases in which pleural levels of ADA are below 47 U/l, pleural fluid cytology is negative and, in the absence of a positive basis for some other diagnosis, neoplasia is suspected.
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