TABLE 2

Pleural fluid analysis and rationale

Blood cell count and differentialA neutrophilic-predominant pleural effusion would make bacterial infection the most likely cause
Lymphocytic pleural effusion is mainly encountered in conditions like rheumatoid arthritis-associated pleural effusion and fungal or tuberculosis-associated pleural effusion
Cholesterol levelCholesterol levels > 60 mg/dL are seen in exudative pleural effusion
CytologyPathologic analysis of pleural fluid allows for detection of malignancy
Culture and Gram stainAllows for the speciation of the pathogenic organism
Culture for bacteria, fungal, and acid-fast bacilli can be sent
Antimicrobial resistance can be determined by sensitivity data
Amylase levelElevated levels seen in acute pancreatitis-associated exudative pleural effusion
Triglyceride levelElevated (> 110 mg/dL) in chylothorax
Bilirubin levelElevated in biliothorax
Albumin levelPleural fluid albumin and serum albumin gradient allow for the determination of pseudoexudative and exudative effusions in the setting of diuretic use
HematocritPleural fluid hematocrit > 50% is pathognomonic for hemothorax
pHLow pH pleural fluid seen in empyema or rheumatoid arthritis-associated pleural effusion
Light criteriaDifferentiates between exudative or transudative pleural effusion; if at least 1 of the following criteria is met, the pleural effusion is exudative:

  1. Ratio of pleural fluid protein to serum protein concentration > 0.5

  2. Pleural fluid LDH greater than two-thirds of the upper limit of normal for serum LDH

  3. Ratio of pleural fluid LDH to serum LDH concentration > 0.6

  • LDH = lactate dehydrogenase

  • Data from references 2–9.