A 65 year old male presents to the clinic with complaints of weight loss, fatigue, night sweats, and muscle pain. His overall health history is negative. He takes medications for mild hypertension. The patient was never married and has no children. Initial labs today include a complete blood count (CBC), complete chemistry profile, urinalysis, lipid profile, and thyroid profile. He also undergoes a tuberculosis skin test and electrocardiogram. What additional test is essential at this time?