Subinternship Medicine · Year 4 · from Subinternship Medicine
Case 3: Thoracentesis with Light's Criteria Application
Clinical Image
Source: Wikimedia Commons - Pleural Effusion - CC BY-SA 4.0
Case Presentation
A 55-year-old woman with history of breast cancer (in remission for 5 years) presents with progressive dyspnea over 2 weeks. Chest X-ray reveals a large left pleural effusion. She has no fever, cough, or chest pain. She has no known heart failure, liver disease, or kidney disease. Given the unclear etiology and cancer history, diagnostic and therapeutic thoracentesis is planned. Using ultrasound, the sub-intern identifies the effusion, confirms depth of approximately 3 cm from skin to fluid, and marks the optimal entry site. The patient sits upright leaning forward over a bedside table. Using sterile technique, local anesthesia is infiltrated, and the needle is inserted just above the rib (to avoid the neurovascular bundle along the lower rib margin). Approximately 1.2 liters of serosanguineous fluid is removed (stopping before 1.5 L to avoid re-expansion pulmonary edema). She reports immediate improvement in breathing. Fluid analysis: protein 5.2 g/dL (serum 7.0 g/dL, ratio 0.74), LDH 420 U/L (serum 180 U/L, ratio 2.3), glucose 45 mg/dL. Light's criteria indicate an exudative effusion (meets 2 of 3 criteria: protein ratio greater than 0.5 and LDH ratio greater than 0.6). Cytology returns positive for adenocarcinoma consistent with breast primary. She is diagnosed with malignant pleural effusion from recurrent breast cancer.
Key Learning Points
- Light's criteria distinguish exudate from transudate: exudate if ANY of the following are met: pleural/serum protein greater than 0.5, pleural/serum LDH greater than 0.6, or pleural LDH greater than 2/3 upper limit of normal for serum
- Transudates result from hydrostatic/oncotic imbalances (heart failure, cirrhosis, nephrotic syndrome); exudates result from pleural inflammation or impaired lymphatic drainage (infection, malignancy, PE)
- Limit therapeutic thoracentesis to 1.5 L to prevent re-expansion pulmonary edema
- Enter above the rib to avoid the intercostal neurovascular bundle; use ultrasound to confirm fluid location and depth, reducing pneumothorax risk