C-peptide levels are increased with renal failure. (C-peptide is normally excreted by the kidneys.) Instances of insulinoma have been described in which proinsulin was increased but insulin and C-peptide were not.

Patient should fast for 14-16 hours for basal values. This test may exhibit interference when sample is collected from a person who is consuming a supplement with a high doseof biotin (also termed as vitamin B7 or B8, vitamin H, or coenzyme R).