A 68-year-old woman with a history of rheumatoid arthritis presents with acute dyspnea and pleuritic chest pain. Pleural fluid analysis shows: LDH 450 IU/L (serum 200), protein 4.2 g/dL (serum 6.8), pH 6.8, glucose 25 mg/dL, cholesterol 80 mg/dL. Which finding most reliably distinguishes a rheumatoid pleural effusion from an empyema?