A pleural effusion is a common medical problem. Despite several diagnostic tests, 15-20% of pleural effusions go undiagnosed. The aim of this study was to evaluate the clinical characteristics and prognosis of a lymphocyte dominant exudative pleural effusion with a low adenosine deaminase (ADA), low carcinoembryonic antigen (CEA), negative cytology and negative acid fast bacilli (AFB) smear.

From Jan 2000 to Aug 2001, 43 patients with lymphocyte dominant exudative pleural effusions whose AFB smear and cytologic exam were negative, their pleural fluid ADA level was < 40 IU/L, and their CEA level was < 10 ng/mL were enrolled in this study. A retrospective analysis of the patients' medical records was carried out.

Among 31 of the 43 cases (72%), probable underlying diseases causing the pleural effusion were identified: 21cases of malignant diseases, 4 cases of liver cirrhosis, 2 cases of pulmonary tuberculosis, 1 case of end stage renal disease, 1 case of a chylothorax, 1 case of a post-CABG (coronary artery bypass graft) state, 1 case of a pulmonary embolism. No clinically suspected etiology was identified in the remaining 12 cases (28%). Of these 12 pleural effusions, 7 cases spontaneously resolved, 2 effusions resolved with antibiotics, and the other 2 cases were persistent.

Lymphocyte dominant exudative pleural effusions with a low ADA, low CEA, negative cytological exam, and negative AFB smear, but without a definite cause might have a benign course and clinicians can observe them with attention.