MRCP2-4360

A 31-year-old man presents to the acute medical unit with sudden onset of shortness of breath. Upon admission, blood cultures reveal the presence of Staphylococcus aureus. During examination, a pansystolic murmur is heard loudest over the left sternal edge. A chest x-ray confirms the presence of a cavitating mass in the right lung with an associated pleural effusion. You perform a pleural tap and obtain the following results:

pH 7.15
Protein 42 g/l
Appearance Serous

What test can be conducted to determine if the effusion is an empyema?