## Diagnostic Approach to Pleural Effusion **Key Point:** Thoracentesis with pleural fluid analysis and cytology is the gold standard investigation for characterizing pleural effusions and establishing etiology, particularly in suspected malignancy. ### Why Thoracentesis is the Investigation of Choice Thoracentesis provides: 1. **Direct fluid sampling** — allows biochemical analysis (protein, LDH, glucose), cell count, and microscopy 2. **Cytological examination** — detects malignant cells with sensitivity 40–65% for malignant effusions 3. **Microbiological culture** — identifies infectious organisms if empyema suspected 4. **Diagnostic yield** — combined with imaging, achieves diagnosis in >90% of cases ### Pleural Fluid Analysis Parameters | Parameter | Exudate | Transudate | | --- | --- | --- | | Protein (g/dL) | >3.0 | <3.0 | | LDH (IU/L) | >200 | <200 | | Pleural/serum protein ratio | >0.5 | <0.5 | | Pleural/serum LDH ratio | >0.6 | <0.6 | | Cell count | Variable | <1000 WBC/μL | **High-Yield:** Light's criteria (protein ratio, LDH ratio, or absolute LDH >2/3 upper limit of normal serum) differentiate exudates from transudates with >90% accuracy. ### Role of Imaging in Pleural Effusion **Clinical Pearl:** While CT and ultrasound help localize effusions and guide needle placement, they do NOT provide tissue diagnosis. Imaging is complementary, not diagnostic, for determining etiology. **Mnemonic: CUFF** — Causes of exudative pleural effusion: - **C**arcinoma (malignancy) - **U**retemia, **U**nderlying lung disease (pneumonia, PE, infarction) - **F**ailure (cardiac), **F**ungal/TB ### Ultrasound-Guided Pleural Biopsy Plural biopsy is reserved for: - Suspected TB (higher yield than fluid cytology alone) - Suspected mesothelioma - Non-diagnostic thoracentesis in high clinical suspicion It is NOT first-line for malignancy confirmation. [cite:Harrison 21e Ch 297] 
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