Heart Failure is the Most Likely Diagnosis
This pleural effusion is a transudate caused by heart failure, despite the LDH ratio meeting Light's criteria for an exudate. 1, 2
Application of Light's Criteria
The patient's values reveal a diagnostic dilemma:
- Protein ratio = 0.5 (pleural 35 ÷ serum 60): Does NOT meet exudate criteria (needs >0.5) 1, 2
- LDH ratio = 2.0 (pleural 200 ÷ serum 100): DOES meet exudate criteria (>0.6) 1, 2
- Light's criteria classify this as an exudate because one criterion is met, even though the protein ratio is at the borderline 1
Why This is Actually Heart Failure (A "False Exudate")
The European Respiratory Society specifically addresses this scenario: 25-30% of cardiac and hepatic transudates are misclassified as exudates by Light's criteria. 2 This case demonstrates the classic pattern:
- Protein ratio of exactly 0.5 strongly suggests a transudate that barely crosses the threshold 1, 3
- LDH ratio of 1.7 is only modestly elevated, not the markedly high values seen in true exudates 1
- Heart failure accounts for 29-53.5% of all pleural effusions and is the most common cause of transudative effusions 2
Excluding Other Diagnoses
Tuberculosis is definitively ruled out because:
- TB always causes exudative effusions with BOTH high LDH ratios (typically >0.6) AND high protein ratios (>0.5) 2, 4
- The protein ratio of 0.5 excludes TB 4
- TB typically shows LDH/ADA ratios <14.2, whereas this patient would have much higher ratios given the modest LDH elevation 5, 6
Liver cirrhosis causes transudates (3-10% of cases) but:
- Would show BOTH protein and LDH ratios below exudate thresholds 2, 3
- The LDH ratio of 2.0 makes cirrhosis less likely 3
Hyponatremia is not a cause of pleural effusion but rather a laboratory finding that may accompany various conditions 2
Confirming the Diagnosis
When Light's criteria provide borderline results with high pre-test probability for heart failure, calculate the serum-pleural albumin gradient. 1, 2 A gradient >1.2 g/dL correctly reclassifies approximately 80% of these "false exudates" as transudates from heart failure 1
Alternatively, NT-proBNP levels >1500 μg/mL in serum or pleural fluid have 92-94% sensitivity and 88-91% specificity for diagnosing heart failure as the cause of pleural effusion. 1, 2