Low yield of microbiologic studies on pleural fluid specimens

Area of Science:

• Medical Microbiology
• Pulmonary Medicine
• Diagnostic Pathology

Background:

• Pleural effusions of unknown etiology often prompt microbiologic cultures of pleural fluid.
• The clinical utility of routine bacterial, mycobacterial, and fungal cultures for pleural effusions has not been well-established.

Purpose of the Study:

• To assess the diagnostic yield of routine microbiologic cultures and smears of pleural fluid specimens.
• To evaluate the effectiveness of current practices in identifying pathogens in pleural effusions.

Main Methods:

• Retrospective review of 525 patients undergoing diagnostic thoracentesis at a tertiary care center.
• Analysis of bacterial, mycobacterial, and fungal cultures and Gram, KOH, and acid-fast smears from pleural fluid.
• Exclusion of likely contaminants to determine true pathogen identification rates.

Main Results:

• Only 1.4% of 1,320 pleural fluid cultures yielded true pathogens, identifying them in 3.2% of patients with cultures.
• Positive yields were low across aerobic bacteria (2.3%), anaerobic bacteria (1.2%), and fungi (1.4%); mycobacterial cultures were negative.
• Microbiologic smears had a low positive yield (1.3%), with Gram stains showing 2.5% positivity.
• Outpatient and free-flowing effusions demonstrated particularly low rates of true-positive results (0.8% and 1.1%, respectively).

Conclusions:

• The diagnostic yield of routine microbiologic testing of pleural fluid is low.
• Testing is least effective in outpatient settings and for free-flowing effusions.
• Selective ordering of pleural fluid cultures and smears is warranted to improve efficiency and resource utilization.