This document provides guidance on evaluating and diagnosing pleural effusions based on laboratory tests of pleural fluid obtained from thoracentesis. It outlines the indications for thoracentesis and initial labs to differentiate transudative from exudative effusions and identify the cause. For exudates, tests include cell count, cultures, cytology and others depending on suspected infection or cancer. Findings that suggest specific diagnoses like transudate, empyema or hemothorax are also provided.

1. 1. See Also
a. Pleural Effusion
b. Pleural Effusion Causes
c. Thoracentesis
2. Indications
a. Evaluation of Pleural Effusion
3. Preparation
a. Based on fluid obtained during Thoracentesis
b. Initial labs: Differentiate transudate from exudate
i. Pleural glucose
ii. Lactate Dehydrogenase (LDH)
1. Pleural LDH
2. Serum LDH
iii. Protein
1. Serum Protein
2. Pleural protein
iv. Consider Cholesterol
v. Consider NT-proBNP (suggests CHF if >1500 pg/ml)
c. Initial labs: Identify cause (especially exudate)
i. Cell count with differential
ii. Gram Stain and culture
iii. Pleural fluid cytology (Test Sensitivity: 60%)
1. False negatives in Mesothelioma, Sarcoma, Lymphoma
2. Consider repeat Thoracentesis if non-diagnostic
d. Other labs: Infection suspected
i. PCR for Streptococcus Pneumoniae
ii. Infection suspected despite non-purulent fluid
1. Pleural fluid pH
iii. Tuberculosis suspected
1. Adenosine deaminase (Sensitive for Tuberculosis)
2. Interferon (alternative to Adenosine deaminase)
3. PCR for Mycobacterium tuberculosis
4. Culture for Mycobacterium tuberculosis
5. AFB smears are rarely positive
e. Other labs: Cancer suspected
i. Tumor Markers
ii. Pleural amylase (Cancer, pancreatic source)
iii. Triglycerides (Lymphoma, trauma)
4. Findings: Transudate
a. See Transudate Pleural Effusion Causes
b. Clear fluid
c. Protein < 3 g/dl
d. Lactate Dehydrogenase (LDH) <200 IU/L
e. Glucose >60 mg/dl
f. White Blood Cell Count < 1000/ml
5. Findings: Exudate
a. See Exudate Pleural Effusion Causes
b. False positive in CHF after Diuretics
i. Finding: Serum Protein - Pleural protein >3.1 g/dl
ii. Finding: Serum Albumin - pleural albumin >1.2 g/dl
c. Clear, cloudy or bloody fluid
d. Protein
i. Pleural protein > 3 g/dl
ii. Pleural protein to Serum Protein ratio >0.5
e. Lactate Dehydrogenase
i. Pleural LDH > 200 IU/L
ii. Pleural LDH > 2/3 serum LDH upper normal limit
iii. Pleural LDH to serum LDH ratio >0.6
f. Glucose < 60 mg/dl

2. g. White Blood Cell Count > 1,000/ml
h. Cholesterol >45 to 60 mg/dl
6. Findings: Empyema
a. See Empyema Pleural Effusion Causes
b. White Blood Cell Count > 10,000 cells/cu mm
c. Gram Stain
d. Pleural fluid culture positive
e. Glucose > 40 mg/dl
f. pH < 7.2
g. Additional tests to consider when infection suspected
i. Consider PCR for Streptococcus Pneumoniae
ii. Tuberculosis testing is described above
7. Findings: Uniformly bloody effusion
a. See Bloody Pleural Effusion Causes
b. Fluid Hematocrit >1% (hemothorax if >50% Hematocrit)
c. Red Blood Cell Count >100,000 per mm3
8. References
a. Light (2002) N Engl J Med 346:1971
b. Medford (2005) Postgrad Med J 81(961):702
c. Porcel (2006) Am Fam Physician 73:1211
d. Rabman (2005) Br Med Bull 72:31