Distinguishing Transudative from Exudative Pleural Fluid
Measure pleural fluid protein and LDH, then apply Light's criteria: an effusion is an exudate if it meets any one of these three thresholds: pleural fluid/serum protein ratio >0.5, pleural fluid/serum LDH ratio >0.6, or pleural fluid LDH >0.67 of the upper limit of normal serum value. 1, 2
Initial Clinical Assessment
Before biochemical testing, evaluate the clinical context to identify obvious transudates:
- Do not aspirate bilateral effusions if the clinical picture strongly suggests transudate (e.g., left ventricular failure, hypoalbuminemia, dialysis) unless atypical features exist or the effusion fails to respond to therapy 1
- Clinical assessment alone correctly identifies transudates in many cases—in one series, all 17 transudates were accurately predicted by clinical evaluation without fluid analysis 1
- Common transudate causes: heart failure (53.5% of bilateral effusions), liver cirrhosis, nephrotic syndrome, hypoalbuminemia 1
Biochemical Differentiation: Light's Criteria
Light's criteria remain the gold standard with the following performance characteristics 1, 2:
- Sensitivity: 98%
- Specificity: 72%
- Positive likelihood ratio: 3.5
- Negative likelihood ratio: 0.03
The Three Criteria (Meeting ANY ONE = Exudate):
- Pleural fluid/serum protein ratio >0.5 1, 2
- Pleural fluid/serum LDH ratio >0.6 1, 2
- Pleural fluid LDH >0.67 of upper limit of normal serum value 1, 2
Practical Sampling Approach
- Obtain pleural fluid via fine bore (21G) needle with 50 ml syringe 1
- Send samples for: protein, LDH (to clarify borderline protein values), pH, Gram stain, AAFB stain, cytology, and microbiological culture 1
- Simultaneously obtain serum samples for protein and LDH to calculate ratios 3
Borderline Cases and Alternative Methods
When Pleural Fluid Protein is 25-35 g/L:
Use Light's criteria to differentiate accurately between transudates and exudates in this borderline range 1
If Serum Sample Unavailable:
Use an "or" rule combining:
- Pleural fluid LDH >67% upper limit of normal serum LDH AND
- Pleural fluid cholesterol >55 mg/dL 1
This combination has discriminative capacity equivalent to Light's criteria 1, 4
Addressing Misclassification:
- Light's criteria may misclassify 25-30% of cardiac and liver transudates as exudates due to the criteria being designed to maximize exudate detection 1
- If a patient clinically appears to have a transudate but meets exudative criteria, calculate the serum-to-pleural fluid albumin gradient: if >1.2 g/dL, the effusion is actually transudative 5
Common Pitfalls to Avoid
- Don't skip Light's criteria for borderline protein values (25-35 g/L)—this is precisely when they are most needed 1
- Don't rely on protein alone when serum protein is abnormal—ratios are essential 1
- Don't forget that some conditions cause either transudate or exudate (e.g., non-expansile lung, chylothorax, superior vena cava syndrome) 1
- Don't assume all bilateral effusions are transudates—malignancy causes 18% of bilateral effusions 1
Clinical Implications After Classification
If Transudate:
- Treat the underlying cause (heart failure, cirrhosis, nephrotic syndrome) 1
- Further pleural investigation usually unnecessary 1
- Most transudates (>80%) are due to heart failure 1
If Exudate:
- Pursue additional diagnostic workup for pneumonia, malignancy, tuberculosis, or pulmonary embolism—these account for most exudative effusions 1, 3
- Consider additional pleural fluid tests: pH, glucose, amylase, cytology, adenosine deaminase 2, 6, 5
- Refer to chest physician if diagnosis remains unclear after initial fluid analysis 1