Exudative Pleural Effusion - Most Likely Tuberculosis
This patient has an exudative pleural effusion based on Light's criteria (LDH ratio 2.0, protein ratio 0.58), and among the answer choices provided, tuberculosis (Option C) is the most likely diagnosis as it characteristically causes exudates, while heart failure and liver cirrhosis typically cause transudates. 1, 2
Application of Light's Criteria
The biochemical analysis clearly demonstrates an exudate:
- Pleural fluid LDH/Serum LDH ratio = 200/100 = 2.0 (>0.6 indicates exudate) 1
- Pleural fluid protein/Serum protein ratio = 35/60 = 0.58 (>0.5 indicates exudate) 1
- Pleural fluid LDH = 200 (>2/3 upper limit of normal serum LDH indicates exudate) 2, 3
The British Thoracic Society guidelines establish that meeting any one of these three criteria classifies the effusion as an exudate, and this patient meets all three criteria 1. The pleural fluid protein of 35 g/L is also above the classical exudate threshold of >30 g/L 1.
Differential Diagnosis Analysis
Why Not Heart Failure (Option B)?
- Heart failure is the most common cause of transudative effusions, accounting for 29-53.5% of all pleural effusions 2
- Transudates have protein ratio <0.5 and LDH ratio <0.6 4
- This patient's ratios (protein 0.58, LDH 2.0) definitively exclude a simple transudate 1
- Important caveat: 25-30% of cardiac transudates can be misclassified as exudates by Light's criteria, particularly in patients on diuretics 2. However, the markedly elevated LDH ratio of 2.0 (far exceeding the 0.6 threshold) makes this unlikely 1
Why Not Liver Cirrhosis (Option D)?
- Liver cirrhosis causes transudative effusions in approximately 3-10% of cases 2
- The exudative pattern with elevated LDH and protein ratios excludes uncomplicated hepatic hydrothorax 4
Why Tuberculosis (Option C)?
- Tuberculosis always causes exudative effusions with high LDH and protein ratios 2, 5
- Tuberculous pleural effusions characteristically show fluid-to-serum protein ratios exceeding 0.5 and LDH ratios exceeding 0.6 5
- In a study of 44 tuberculous pleurisy patients, all demonstrated exudative characteristics with lymphocyte predominance 5
- Tuberculous effusions typically have markedly elevated LDH levels, and higher pleural LDH correlates with subsequent pleural thickening 6
Why Not Hyponatremia (Option A)?
- Hyponatremia is a laboratory finding, not a diagnosis causing pleural effusion 1
- This option is a distractor and clinically irrelevant to the pleural fluid analysis presented
Clinical Pearls
Common pitfall: Do not assume all cardiac patients have transudates. Patients on chronic diuretic therapy can develop "pseudo-exudates" where Light's criteria misclassify a cardiac transudate as an exudate 2. In such cases, a serum-effusion albumin gradient >1.2 g/dL or NT-proBNP >1500 μg/mL would reclassify the effusion as transudate 2.
Key discriminator: The LDH ratio of 2.0 is markedly elevated (more than 3-fold above the 0.6 threshold), strongly suggesting true exudative pathology rather than diuretic-induced misclassification 1, 3.