A 68-year-old woman with a 40-pack-year smoking history presents with unilateral pleural effusion on imaging. Thoracentesis reveals an exudative effusion with LDH 450 IU/L (serum LDH 180 IU/L), protein 4.2 g/dL (serum protein 6.8 g/dL), and negative bacterial culture. Cell count shows 60% lymphocytes. Which is the most appropriate next investigation to establish the diagnosis?