Four adult patients hospitalized with respiratory symptoms undergo thoracentesis for newly… — USMLE Step 3

Four adult patients hospitalized with respiratory symptoms undergo thoracentesis for newly discovered pleural effusions. Based on Light's criteria and the accompanying specialized fluid analyses, match each patient's pleural fluid (PF) profile to the most likely underlying etiology.

PF/Serum Protein: $0.3$ , PF/Serum LDH: $0.4$ , PF WBC: $400/\mu L$ Heart failure exacerbation
PF/Serum Protein: $0.8$ , PF/Serum LDH: $0.8$ , PF Glucose: $20$ mg/dL, PF WBC: $45,000/\mu L$ , pH: $6.9$ Empyema
PF/Serum Protein: $0.7$ , PF/Serum LDH: $0.9$ , PF WBC: $1,500/\mu L$ ( $85\%$ lymphocytes), Adenosine deaminase (ADA): $55$ U/LTuberculous pleurisy
PF/Serum Protein: $0.6$ , PF/Serum LDH: $0.7$ , PF Glucose: $15$ mg/dL, Rheumatoid factor: positiveRheumatoid arthritis-associated effusion

Answer

The transudative profile matches heart failure; the purulent, low-pH profile matches empyema; the lymphocytic profile with elevated ADA matches tuberculous pleurisy; and the exudative profile with extremely low glucose matches rheumatoid arthritis-associated effusion.

Correct matching requires sequential application of Light's criteria to separate transudates (heart failure) from exudates (empyema, TB, rheumatoid arthritis), followed by interpretation of specific fluid markers (pH, ADA, cell counts, RF) to distinguish among the exudative etiologies.

Step-by-Step Solution

Apply Light's criteria to each pleural fluid profile.

The first profile is a transudate (protein ratio

\le 0.5

, LDH ratio

\le 0.6

). The other three profiles are exudates.

Differentiating transudates from exudates narrows down the underlying pathophysiologic mechanism (hydrostatic/oncotic versus inflammatory/malignant).

Identify the cause of the transudative effusion.

The transudative fluid is matched with heart failure exacerbation.

Heart failure causes fluid accumulation via increased pulmonary capillary hydrostatic pressure, without capillary leakage of large proteins or LDH.

Analyze the specific biochemical markers in the three exudative fluids.

The high-neutrophil/low-pH fluid indicates bacterial infection (empyema). The high-lymphocyte/high-ADA fluid indicates tuberculosis. The low-glucose/RF-positive fluid indicates rheumatoid pleuritis.

While exudates share similar Light's criteria, specific markers like cell differential, pH, glucose, and specialized tests (ADA, RF) allow for precise etiologic diagnosis.

Key Concept

Interpretation of pleural fluid using Light's criteria and etiology-specific biochemical markers.

Estimated Time:1m 0s

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