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Difficulty: MediumInterpreting Laboratory Results

A hospital medicine team is evaluating four distinct patients who have recently presented with symptomatic pleural effusions. Thoracentesis is performed on each patient to establish a diagnosis, and the pleural fluid analyses are sent to the laboratory. Match each specific pleural fluid laboratory profile to its most likely underlying diagnostic etiology.

  • Pleural fluid/serum protein ratio 0.3; Pleural fluid/serum LDH ratio 0.4; Leukocyte count 800/mm³ (predominantly mononuclear)Congestive heart failure
  • Pleural fluid/serum protein ratio 0.7; Pleural fluid/serum LDH ratio 0.9; Pleural fluid glucose 15 mg/dL; pH 7.05Rheumatoid pleurisy
  • Pleural fluid/serum protein ratio 0.6; Pleural fluid/serum LDH ratio 0.8; Amylase 1,500 U/L; pH 6.90Esophageal rupture
  • Pleural fluid/serum protein ratio 0.55; Pleural fluid/serum LDH ratio 0.7; Leukocyte count 6,000/mm³ (85% lymphocytes); Adenosine deaminase (ADA) 65 U/LTuberculous pleurisy

Answer

The transudative fluid matches congestive heart failure; the low-glucose exudate matches rheumatoid pleurisy; the high-amylase, low-pH exudate matches esophageal rupture; and the lymphocytic, high-ADA exudate matches tuberculous pleurisy.
Each specific etiology of pleural effusion generates a distinct biochemical profile that can be matched by correctly applying Light's criteria and analyzing disease-specific fluid markers.

Step-by-Step Solution

1
Apply Light's criteria to classify each pleural fluid profile as a transudate or exudate.
The first profile is identified as a transudate, while the remaining three profiles are identified as exudates.
Determining whether a fluid is a transudate or exudate fundamentally narrows the differential diagnosis between systemic processes (hydrostatic/oncotic) and local pleural pathologies.
2
Evaluate the transudate profile.
It corresponds to congestive heart failure.
Congestive heart failure is the most common cause of a transudative effusion, lacking significant cellularity or drastic changes in pH and glucose.
3
Analyze the exudate with exceptionally low glucose (15 mg/dL) and low pH.
It matches rheumatoid pleurisy.
Rheumatoid pleurisy is known for classically producing profoundly low pleural fluid glucose levels, often below 30 mg/dL, alongside a low pH and high LDH.
4
Evaluate the exudate with elevated amylase and markedly low pH.
It matches esophageal rupture.
Esophageal perforation spills salivary amylase and acidic gastric contents into the pleural space, leading to the distinct combination of high amylase and severe acidosis (pH < 7.0).
5
Analyze the lymphocyte-predominant exudate with elevated ADA.
It matches tuberculous pleurisy.
Elevated adenosine deaminase (ADA) in a lymphocytic exudate strongly supports the diagnosis of pleural tuberculosis.

Key Concept

Applying Light's criteria to classify pleural effusions followed by interpreting specific biochemical markers (glucose, pH, amylase, ADA) to establish a definitive etiology.
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Interpreting Laboratory Results

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