C-peptide is a short protein chain released from the pancreas in equal amounts to insulin when proinsulin is cleaved into insulin. Because C-peptide and insulin are produced in a 1:1 ratio, measuring C-peptide is the most reliable way to assess how much insulin the pancreas is actually producing — even in people taking exogenous (injected) insulin, which C-peptide cannot detect.

C-peptide testing is used to distinguish type 1 from type 2 diabetes, assess residual beta cell function in people with known diabetes, investigate hypoglycaemia (low blood sugar), and evaluate candidates for pancreatic surgery or islet cell transplant. Low C-peptide indicates the pancreas is producing little insulin; high C-peptide indicates excess insulin production, as seen in insulinoma or insulin resistance.