A 73-year-old male presents with depression and low energy f…
A 73-year-old male presents with depression and low energy for the past month. He has no significant medical issues. Memory and cognition are intact. You prescribe his paroxetine 20mg PO QAM for depression. Two weeks later he is brought into the emergency room by his family with disorientation, nausea and weakness. Which laboratory test is most likely to establish his diagnosis?