Test	Test value	Suggested diagnosis	Comments
Amylase	> Upper limit of normal	Malignancy (<20 percent), pancreatic disease, esophageal rupture5,16	Obtain when esophageal rupture or pancreatic disease is suspected. The amylase in malignancy and esophageal rupture is of the salivary type.
Cholesterol	> 45 to 60 mg per dL (1.16 to 1.55 mmol per L)	Any condition causing an exudate	Measure if chylothorax or pseudochylothorax is suspected. This parameter taken alone misclassifies 10 percent of exudates and 20 percent of transudates.13
Culture	Positive	Infection	Obtain in all parapneumonic pleural effusions because a positive Gram stain or culture should lead to prompt chest tube drainage.14,15
Hematocrit fluid to blood ratio	≥0.5	Hemothorax	Obtain when pleural fluid is bloody. Hemothorax most often originates from blunt or penetrating chest trauma.
Interferon*	Different cutoff points	Tuberculosis17	Consider when ADA is unavailable or nondiagnostic and tuberculosis is suspected.
NT-proBNP	> 1,500 pg per mL	Heart failure18	If available, consider testing when heart failure is suspected and exudate criteria are met.19
pH	< 7.20	Complicated parapneumonic effusion or empyema, malignancy (< 10 percent), tuberculosis (< 10 percent), esophageal rupture5	Obtain in all nonpurulent effusions if infection is suspected. A low pleural fluid pH indicates the need for tube drainage only for parapneumonic pleural effusions.
Polymerase chain reaction †	Positive	Infection20,21	Consider when infection is suspected. Sensitivity of polymerase chain reaction to detectMycobacterium tuberculosis in pleural fluid varies from 40 to 80 percent and is lower in patients with negative mycobacterial cultures.
Triglycerides	> 110 mg per dL (1.24 mmol per L)	Chylothorax	Obtain when pleural fluid is cloudy or milky. Chylothorax is caused by lymphoma or trauma. Not all chylous pleural effusions appear milky white or whitish.
Tumor markers‡	Different cutoff points	Malignancy	Consider when malignancy is suspected and thoracoscopy is being considered. Except for telomerase activity,22 individual tests tend to have low sensitivity (< 30 percent) when looking for the utmost specificity.23,24