A C-peptide test is a regular blood test which is carried out to determine the level of insulin the body is producing. The test is essentially conducted to determine whether the concerned subject is having type 1or type 2 diabetes or whether you have resistance to insulin. This test is released at the same time as insulin. Each molecule of insulin that gets produced there is a molecule of c-peptide. C-peptide does not itself influence blood sugar. It is a useful marker of insulin production as c-peptide tends to remain longer in the blood than insulin.