The pancreas of a person with type 1 diabetes is unable to produce adequate insulin, and, therefore, they will usually have a decreased level of C-peptide.
C-peptide levels in type 2 patients are normal or higher than normal.
The most common MODY syndrome may also have normal fasting C-peptide results because the flaw in this case is in the secretion of insulin in response to rising glucose and fasting secretion is still near normal. Their postprandial C-peptide however is below normal with elevated blood glucose.
