Why would i need an insulin c-peptide measurement?

Insulin production: C-peptide is usually measured to see how much Insulin a patient is producing, specially when he is already on insulin. Proinsulin is broken off into c-peptide and Insulin in our body, therefore if you have detectable amounts of c-peptide despite receiving Insulin you are still making some insulin, if your c-peptide is low you will need Insulin to metabolize your sugar.