A 42-year-old man with a 3-year history of Addison disease presents with acute onset severe hypotension (70/45 mmHg), fever, and confusion. His cortisol level is 8 μg/dL (normal 8–20 μg/dL at 8 AM), and ACTH is 520 pg/mL. Which laboratory finding would most reliably distinguish an acute adrenal crisis in Addison disease from an acute adrenal crisis secondary to pituitary apoplexy?