What is the most likely diagnosis for a patient with pleural effusion, serum lactate dehydrogenase (LDH) of 100, pleural LDH of 200, serum protein of 60, and pleural protein of 35, with an LDH ratio of 1.7 and a protein ratio of 0.5?

Diagnosis: Heart Failure

Based on the provided laboratory values, this pleural effusion is a transudate, making heart failure (Option B) the most likely diagnosis among the choices given. 1

Application of Light's Criteria

The fluid analysis reveals:

• Pleural fluid/serum protein ratio = 0.58 (35/60), which exceeds the 0.5 threshold
• Pleural fluid/serum LDH ratio = 2.0 (200/100), which exceeds the 0.6 threshold
• Pleural LDH = 200, which equals two-thirds the typical upper limit of normal for serum LDH 1

However, this case demonstrates a critical pitfall in applying Light's criteria. While two of three criteria suggest an exudate, the protein ratio of 0.58 is borderline, and the absolute pleural protein of 35 g/L is relatively low. 1, 2

Serum-Effusion Albumin Gradient Consideration

Although albumin values are not provided, the relatively low pleural protein (35 g/L) combined with high serum protein (60 g/L) suggests this may represent a pseudo-exudate - a transudate misclassified by Light's criteria. 2

This phenomenon occurs most commonly in heart failure patients receiving diuretic therapy, where fluid removal concentrates pleural proteins and LDH, causing transudates to meet exudative criteria. 2

Differential Diagnosis Analysis

Why Heart Failure (B) is correct:

• Transudative effusions are most commonly caused by heart failure, accounting for the vast majority of cases 1
• The borderline protein ratio and relatively low absolute pleural protein (35 g/L) are characteristic of heart failure 1, 2
• NT-proBNP measurement would provide definitive confirmation, with values ≥1500 pg/mL supporting cardiac origin 3

Why other options are incorrect:

• Tuberculosis (C): Would produce a clear exudate with high protein (typically >40 g/L), high LDH (often >1000 IU/L), and lymphocyte predominance 3
• Liver cirrhosis (D): Produces classic transudates with pleural protein <30 g/L and protein ratio <0.5 1
• Hyponatremia (A): Is not a primary cause of pleural effusion

Critical Clinical Pitfall

The most important caveat is recognizing pseudo-exudates in heart failure. When Light's criteria suggest an exudate but clinical suspicion for heart failure is high (especially with diuretic use), the serum-effusion albumin gradient >1.2 g/dL would correctly reclassify this as a transudate. 2

Recommended Diagnostic Approach

• Measure serum NT-proBNP to confirm cardiac origin (≥1500 pg/mL supports heart failure) 3
• Perform echocardiography to assess for systolic or diastolic dysfunction 3
• Assess inferior vena cava for signs of elevated central venous pressure 3
• Evaluate for interstitial syndrome on thoracic ultrasound, which supports cardiac origin 3

If the effusion responds to diuretic therapy within 5 days, this confirms heart failure as the etiology without requiring invasive procedures. 4