Low yield of microbiologic studies on pleural fluid specimens

Background: It is generally recommended that pleural fluid samples from pleural effusions of unknown cause be cultured for bacteria, mycobacteria, and fungi. However, the utility of this practice has been not been adequately assessed. Design: Retrospective review. Setting: Tertiary care, referral medical center. Patients: Five hundred twenty-five patients undergoing diagnostic thoracentesis at Mayo Medical Center, Rochester, MN, over a 12-month period from July 1, 2001, to June 30, 2002. Interventions: None. Measurements and results: Among 525 patients undergoing diagnostic thoracenteses, 476 patients (91%) had one or more cultures performed on their pleural fluid specimens. Thirty-nine positive results (3.0% of 1,320 cultures) occurred in 35 of these 476 patients (7.4%). After excluding likely contaminants, true pathogens were identified in only 19 of 1,320 pleural fluid cultures (1.4%) belonging to 15 patients (3.2% of those who had cultures performed on their pleural fluid specimen). These positive results included 2.3% of aerobic bacterial, 1.2% of anaerobic bacterial, 1.4% of fungal, and 0% of mycobacterial cultures. Microbiologie smears performed on these pleural fluid samples included 357 Gram stains, 109 fungal smears (potassium hydroxide), and 232 acid-fast smears with positive yields of 2.5%, 0%, and 0%, respectively. These positive findings represented 1.3% of all smears performed. Of the specimens obtained from outpatient thoracenteses, only one had a true-positive result (0.8%). Only 1.1% (four specimens) of the cultures performed on free-flowing effusions demonstrated true pathogens; three of these four specimens grew fungi. Conclusions: The positive yield of microbiologie smears and cultures on pleural fluid specimens is low, particularly in the outpatient setting and in patients with free-flowing effusions. Microbiologic testing of pleural fluid specimens should be ordered more selectively.