A 52-year-old man from Mumbai presents with progressive dyspnea and right-sided chest pain for 3 weeks. He has a 20-year history of smoking and occupational asbestos exposure. On examination, breath sounds are diminished on the right. Chest X-ray shows a moderate right-sided pleural effusion. Thoracentesis is performed and pleural fluid analysis shows: protein 4.2 g/dL, LDH 245 IU/L, glucose 95 mg/dL, pH 7.4, cell count 850/μL (80% lymphocytes, 15% macrophages, 5% neutrophils). Serum protein is 6.8 g/dL and serum LDH is 180 IU/L. What is the most appropriate next step in the workup?