Pleural Fluid Analysis Approach

Initial Fluid Collection and Gross Examination

Obtain pleural fluid with a 21G needle and 50 mL syringe, placing samples in both sterile vials and blood culture bottles to maximize diagnostic yield for microbiological examination, in patients with suspected pleural effusion 1
Document the appearance and odor of the fluid immediately, to aid in the diagnosis of pleural effusion, in patients undergoing thoracentesis 1, 2
If the fluid appears turbid or milky, centrifuge it: clear supernatant indicates empyema, while persistent turbidity suggests chylothorax or pseudochylothorax, in patients with pleural effusion 1, 2
For bloody fluid, obtain a pleural fluid hematocrit: >50% of peripheral blood hematocrit confirms hemothorax, while <1% indicates insignificant blood contamination, in patients with traumatic or non-traumatic hemothorax 1, 2

Apply Light's criteria when pleural fluid protein is between 25-35 g/L or when serum protein is abnormal, to differentiate exudate from transudate, in patients with pleural effusion 1, 2
Measure pleural fluid cholesterol: >55 mg/dL indicates exudate with diagnostic accuracy equivalent to Light's criteria, in patients with pleural effusion, as recommended by the European Respiratory Society 3, 4
Calculate serum-to-pleural fluid albumin gradient: >1.2 g/dL reclassifies the effusion as transudate, in patients with suspected heart failure or cirrhosis, as recommended by the European Respiratory Society 2, 3
Measure NT-proBNP: >1500 pg/mL indicates heart failure with 93% sensitivity and 93% specificity, in patients with suspected heart failure, as recommended by the European Respiratory Society 3, 5

Measure pleural fluid pH in all non-purulent effusions when infection is suspected, to aid in the diagnosis of complicated parapneumonic effusion or empyema, in patients with pleural effusion 1, 2
pH <7.2 with normal blood pH indicates complicated parapneumonic effusion/empyema, requiring urgent chest tube drainage, in patients with pleural effusion, as recommended by the American Thoracic Society 1, 2, 6
Elevated nucleated cell count with neutrophil predominance suggests parapneumonic effusion or empyema, in patients with pleural effusion, as recommended by the American Thoracic Society 6
Measure pleural fluid adenosine deaminase and/or interferon-gamma, in high tuberculosis prevalence populations, to aid in the diagnosis of tuberculosis, as recommended by the World Health Organization 5

Measure pleural fluid ANA to support diagnosis of lupus pleuritis, in patients with suspected systemic lupus erythematosus, as recommended by the American College of Rheumatology 5
Send pleural fluid for cytology in suspected malignancy, recognizing that only 60% of malignant effusions are diagnosed by cytological examination alone, in patients with pleural effusion, as recommended by the American Thoracic Society 1, 2, 5