What is the most likely diagnosis for a patient with an exudative pleural effusion, serum Lactate Dehydrogenase (LDH) of 100, pleural LDH of 200, serum protein of 60, and pleural protein of 35?

Exudative Pleural Effusion - Most Likely Diagnosis

This patient has an exudative pleural effusion based on Light's criteria, and among the options provided, tuberculosis (Option C) is the most likely diagnosis, as it always presents as an exudate, whereas heart failure and liver cirrhosis typically cause transudates. 1

Classification Using Light's Criteria

The pleural fluid meets criteria for an exudate based on the following calculations: 1, 2

• Pleural fluid/serum protein ratio = 35/60 = 0.58 (>0.5 threshold met for exudate)
• Pleural fluid/serum LDH ratio = 200/100 = 2.0 (>0.6 threshold met for exudate)
• The LDH ratio of 2.0 is well above the 0.6 cutoff required to classify this as an exudate 1

Meeting even one of Light's criteria classifies the effusion as exudative, and this patient meets two criteria definitively. 1, 2

Analysis of Each Answer Option

Option A: Hyponatremia

• Hyponatremia is not a cause of pleural effusion—it is an electrolyte abnormality that may coexist with conditions causing effusions but does not itself produce pleural fluid 1
• This option can be immediately eliminated

Option B: Heart Failure

• Heart failure is the most common cause of pleural effusion overall (29-53.5% of cases), but it characteristically produces a transudate, not an exudate 1
• More than 80% of transudates are due to heart failure 1
• While heart failure patients on diuretics can occasionally be misclassified as exudates (pseudoexudates), this typically occurs when the protein ratio is borderline (0.5-0.6) and the LDH ratio is just above 0.6 1, 2, 3
• This patient's LDH ratio of 2.0 is far too elevated to represent a pseudoexudate from heart failure 1, 2

Option C: Tuberculosis

• Tuberculosis always presents as an exudative effusion and is never transudative 1
• Tuberculosis accounts for approximately 6% of all pleural effusions 1
• Tuberculosis characteristically shows lymphocyte predominance (>50% lymphocytes) in the pleural fluid differential 4
• The elevated LDH ratio of 2.0 is consistent with tuberculosis, which causes significant pleural inflammation 1, 5

Option D: Liver Cirrhosis

• Liver cirrhosis accounts for approximately 3-10% of pleural effusions and characteristically produces a transudate, not an exudate 1, 6
• Cirrhotic effusions occur due to altered hydrostatic forces and movement of ascitic fluid across the diaphragm, not from pleural inflammation 6
• Like heart failure, cirrhosis can occasionally be misclassified as an exudate, but the LDH ratio of 2.0 is incompatible with cirrhosis 1

Key Diagnostic Reasoning

The critical discriminating factor is the markedly elevated LDH ratio of 2.0, which indicates significant pleural inflammation or pathology. 1 Among the answer choices:

• Heart failure and liver cirrhosis produce transudates with normal pleural permeability 1, 6
• When these conditions are misclassified as exudates (pseudoexudates), the serum-effusion albumin gradient >1.2 g/dL or albumin ratio <0.6 can reclassify approximately 80% correctly back to transudates 1, 2
• Tuberculosis is the only option that consistently produces a true exudate with elevated LDH 1

Common Pitfalls to Avoid

• Do not assume all cardiac or cirrhotic effusions are transudates—approximately 25-30% may be misclassified as exudates, particularly in patients on diuretics 1
• However, when the LDH ratio exceeds 2.0, this represents true exudative pathology, not pseudoexudate 1, 2
• Always consider the clinical context: bilateral effusions favor heart failure (53.5% bilateral) while unilateral effusions favor other etiologies 1
• Light's criteria have 98% sensitivity but only 72% specificity for exudates, meaning they can overcall exudates but rarely miss them 1, 2

Answer: C. Tuberculosis