A patient presents with cough and left-sided chest pain. Chest X-ray shows a left pleural effusion. Pleural fluid analysis reveals a milky appearance with a very high LDH level (~800 U/L) and lymphocyte predominance. Cytology shows no malignant cells. AFB smear is negative. Gram stain and culture are negative. D-dimer is 778. What is the most appropriate next diagnostic test?