A 44-year-old man comes to your office because his blood sugars have not been adequately controlled for the last few months. He was diagnosed with diabetes a year ago and for the first 6 months, his blood sugars were well controlled with glipizide 10 mg once daily dose without the need for insulin. However, for the last 3 months, his blood sugar has been hovering. He reports strict adherence to nutrition, and exercise plans and to his medication. He was ‘feeling great’ for a while but now having polyuria, polydipsia, nocturia, and fatigue. His family history is significant for Graves’ disease in his mother and celiac disease in his father. On physical examination, his temperature is 99.4 ⁰ F, heart rate is 62 beats per minute, respiratory rate is 18 breaths per minute, blood pressure is 130/88 mm Hg, and oxygen saturation is 98% on room air, and body mass index is 23 kg/m2. Laboratory findings reveal a hemoglobin A1c level of 11% and a low C-peptide level. Testing for which of the following would help in establishing the diagnosis in this patient?
