Heart Failure is the Most Likely Diagnosis
Based on Light's criteria, this pleural effusion is classified as an exudate (LDH ratio 1.7 meets criteria), but the clinical picture strongly suggests a misclassified transudate due to heart failure, making option B the correct answer. 1, 2
Application of Light's Criteria
The patient's values meet exudative criteria:
- Pleural/serum LDH ratio = 2.0 (200/100) - this exceeds the 0.6 threshold for exudate 1, 2
- Pleural/serum protein ratio = 0.58 (35/60) - this exceeds the 0.5 threshold for exudate 1, 2
However, Light's criteria misclassify approximately 25-30% of cardiac and hepatic transudates as exudates, particularly in patients receiving diuretics. 1, 3
Why Heart Failure is Most Likely
Heart failure accounts for 29-53.5% of all pleural effusions and is the most common cause of transudative effusions (>80% of transudates). 1, 3 The European Respiratory Society guidelines specifically address this common clinical scenario where effusions meet Light's criteria but are actually cardiac in origin. 4, 1
Key Distinguishing Features:
- The protein ratio (0.58) is borderline - just above the 0.5 cutoff, which is characteristic of misclassified cardiac transudates 4, 1
- The LDH ratio (2.0), while elevated, can occur in heart failure patients on diuretics 1, 3
- Right-sided pleural effusions are common in heart failure 1
Ruling Out Other Options
Why Not Tuberculosis (Option C)?
Tuberculosis always causes exudative effusions with markedly elevated LDH and protein ratios, typically with lymphocyte predominance. 1 The borderline protein ratio of 0.58 is inconsistent with TB, which would show much higher values. 1
Why Not Liver Cirrhosis (Option D)?
Liver cirrhosis causes transudates in only 3-10% of pleural effusion cases. 1 While cirrhosis can be misclassified by Light's criteria, it would require additional confirmation with albumin gradient or albumin ratio. 1, 2
Why Not Hyponatremia (Option A)?
Hyponatremia is not a primary cause of pleural effusion - it's a laboratory finding that may accompany various conditions. 1, 2
Recommended Confirmatory Testing
When Light's criteria suggest exudate but heart failure is clinically suspected, the European Respiratory Society recommends:
- Serum-effusion albumin gradient >1.2 g/dL reclassifies the effusion as a transudate 1, 2
- NT-proBNP levels >1500 μg/mL in serum or pleural fluid confirm heart failure with sensitivity of 92-94% and specificity of 88-91% 1, 2
- Albumin ratio (pleural fluid/serum) <0.6 supports cardiac etiology 4, 2
Common Pitfall to Avoid
The most critical error is accepting Light's criteria at face value without considering clinical context. 4, 3 When values are borderline (particularly protein ratio close to 0.5), and the patient has clinical features of heart failure, additional testing with albumin gradient or NT-proBNP is essential to avoid misclassification. 4, 1, 2