A 75 year old man with no past medical history presents with increasing shortness of breath over 6 months. He previously worked at a shipyard where he had significant exposure to asbestos. He has a large right-sided pleural effusion. Pleural aspiration is performed and reveals an exudate but cytopathological examination identifies no malignant cells. CT scanning reveals circumferential pleural thickening but no other abnormalities. Which test is most likely to give a diagnosis?