A C-peptide test measures the level of C-peptide in your blood. C-peptide is a substance produced in the pancreas along with insulin. Measuring C-peptide can help doctors understand how much insulin your body is making. Why is the C-peptide test done? To help diagnose the cause of hypoglycemia (low blood sugar): It can help determine if low blood sugar is due to the body producing too much insulin or other factors. To distinguish between type 1 and type 2 diabetes: In type 1 diabetes, the pancreas produces little to no insulin, so C-peptide levels will be low. In type 2 diabetes, the body may still be producing insulin, so C-peptide levels may be normal or high. To monitor insulin production after a pancreatectomy (removal of the pancreas) or islet cell transplantation: These procedures affect insulin production, and C-peptide levels can help assess how well they are working.