Pleural fluid investigations for pleural infections

Abstract

: Over 1.5 million patients are admitted to hospital with pneumonia in the United States each year. Up to 50% of them can develop a parapneumonic effusion which is associated with higher mortality. The incidence of pleural infection continues to rise, particularly in the elderly and those with comorbidities. Parapneumonic effusions cover a spectrum of presentations from a free-flowing ‘simple parapneumonic effusion’ to a septated ‘complicated parapneumonic effusion’ (CPE) (usually from bacterial pleural invasion) and ‘empyema’ (presence of pleural pus). Pleural infection is defined as either CPE or empyema, and usually requires evacuation of the infected fluid. Laboratory investigations play an essential part of the diagnosis and management of pleural infection. A parapneumonic effusion is typically a neutrophil-rich exudate. Presence of bacteria from culture of the fluid defines pleural infection but conventional culture methods have a low yield. Surrogate pleural fluid markers are often employed to confirm a CPE, including low pleural fluid pH (<7.2) or glucose (<3.3 mmol/L) and elevated lactate dehydrogenase (LDH). Measurement of pleural fluid triglyceride and chylomicrons (for chylothorax) and cholesterol (for pseudochylothorax) may be needed to separate lipid pleural effusions from empyema. Tuberculous pleural effusions usually result from a hypersensitivity pleuritis and are lymphocyte predominant with elevated pleural fluid adenosine deaminase (ADA) and interferon gamma levels. Culture yield of mycobacteria is typically low. Caseating granulomas on pleural tissue biopsy is often considered diagnostic. Common organisms for community-acquired pleural infection include Streptococcus pneumoniae, Streptococcus anginosus group bacteria and Staphylococcus aureus. Hospital-acquired pleural infections have higher mortality and are often polymicrobial which can include S. aureus, Enterobacteriaceae and anaerobes. Antibiotics and evacuation of the infected fluid, usually by chest tube drainage, remain the mainstay of treatment. Intrapleural fibrinolytic and deoxyribonuclease therapy, or occasionally surgical drainage, may be required.