This blood test looks at how well your body's makes insulin. It's used to help diagnose blood sugar disorders. Your body needs the hormone insulin to move sugar through your bloodstream to your cells for energy. A healthy pancreas makes equal amounts of insulin and the protein C-peptide. Measuring C-peptide can show whether you have type 1 or type 2 diabetes. In type 1 diabetes, your body doesn't make any insulin. In type 2 diabetes, either your body doesn't make enough insulin, or your cells can't use it normally. If you have diabetes, the C-peptide test can show how well your treatment is working. The C-peptide test may also be done to find the cause of low blood sugar. Or to check the activity of tumors that make insulin.