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A 78-year-old man was admitted to the hospital 5 days ago for worsening heart failure.

On physical examination at admission, temperature was normal, blood pressure was 150/88 mm Hg, pulse rate was 108/min, and respiration rate was 22/min. There were bibasilar crackles and dullness to percussion at both posterior lung bases. Jugular venous distention, an S₃, and lower extremity edema were present. Chest radiograph revealed cardiomegaly, vascular congestion, and moderate-sized bilateral pleural effusions. He was managed with furosemide and lisinopril. On the fourth hospital day, thoracentesis on the right was performed for further relief of dyspnea.

Pleural fluid analysis demonstrates a pleural fluid to serum lactate dehydrogenase (LDH) ratio of 61%, a pleural fluid LDH that is 46% of the upper limit of serum LDH, and a pleural fluid to serum total protein ratio of 0.51. Pleural fluid cultures and cytology are negative. The serum to pleural fluid total protein gradient is 3.3 g/dL (33 g/L).

Which of the following is the most likely cause of this patient’s pleural effusion?

A: Heart failure
B: Malignancy
C: Pneumonia
D: Pulmonary embolism

MKSAP Answer and Critique

The correct answer is A: Heart failure. This item is available to MKSAP 16 subscribers as item 74 in the Pulmonology and Critical Care section.

The most likely cause of this patient’s pleural effusion is heart failure. This patient presents with classic findings of decompensated heart failure. In this patient, pleural fluid analysis is consistent with an exudate by total protein criteria only (pleural fluid to serum total protein ratio of 0.51), with a transudative lactate dehydrogenase ratio. Pleural fluid differentiation into transudative or exudative categories by modified Light criteria is almost 100% sensitive but only 83% specific for an exudative process, and specificity further declines in the setting of a transudative process with concurrent diuretic therapy, such as in this patient. In this setting, determining the albumin or total protein gradient is useful in confirming the clinical suspicion that the effusions are in fact due to heart failure alone. A serum to pleural fluid albumin gradient greater than 1.2 g/dL (12 g/L) or a serum to pleural fluid total protein gradient greater than 3.1 g/dL (31 g/L) are equally consistent with a transudative process under these circumstances.

Pleural effusions due to malignancy tend to be unilateral with exudative chemical characteristics, and up to two-thirds are lymphocyte predominant. The effusion in this patient is most consistent with a transudate, in which case the lymphocyte predominance is of no clinical significance.

Pneumonia is associated with an exudative pleural effusion, which is not present in this patient. In addition, the absence of fever reduces the probability of a parapneumonic effusion. Although the results of the pleural fluid analysis may increase or decrease the posttest probability that the effusion is exudative, a low clinical suspicion of an exudate should not be affected by borderline test results.

Pleural effusions due to pulmonary embolus are small and unilateral, with 86% resulting in only blunting of the costophrenic angle. Pleural fluid analysis is not helpful in establishing the diagnosis; however, it is almost always consistent with an exudative process.

Key Point

• Heart failure is the most common cause of transudative effusions, but diuresis can cause borderline exudative chemical characteristics; a serum to pleural fluid albumin gradient greater than 1.2 g/dL (12 g/L) or a serum to pleural fluid total protein gradient greater than 3.1 g/dL (31 g/L) is equally consistent with a transudative process under these circumstances.

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