Simply stated, a c-peptide test is effectively a measure of endogenous insulin production. It does so by measuring the levels of this “peptide” (like a protein) that is circulating in the blood. Note that a c-peptide test does not determine what type of diabetes someone has. Because c-peptide is a by-product of insulin creation, which breaks off at the third link from a precursor larger molecule called “proinsulin” to create insulin, it is a good indicator of how much insulin is being produced by the body. Generally, most type 1 patients have very, very low readings, while type 2 patients may range from low to very, very high numbers because insulin resistance causes the beta cells to produce higher-than-normal amounts of insulin. Your best bet in terms of interpreting the results is to discuss them with your doctor.