The C-peptide test is a blood test that measures the level of C-peptide, a molecule produced when the hormone insulin is formed in the pancreas. Unlike insulin, C-peptide is not metabolized by the liver, which makes it a better indicator of insulin production. The test is typically used to determine whether a person with diabetes is producing enough insulin or has been taking too much insulin, which can help guide treatment decisions. The C-peptide test may also be used to distinguish between type 1 and type 2 diabetes, as individuals with type 1 diabetes typically have low or undetectable C-peptide levels due to their inability to produce insulin. In addition, the C-peptide test can be used to monitor the progression of diabetes and to evaluate the function of transplanted pancreatic cells in individuals with type 1 diabetes.