What is a C-peptide test and why is it used for diagnosis?

A C-peptide test measures how much insulin your body is naturally producing. When your pancreas creates insulin, it produces it as a large molecule that splits into two pieces: one piece is insulin, and the other is C-peptide. They are released into the bloodstream in exactly equal amounts. Because insulin doesn't stay in the blood very long, measuring C-peptide provides a much more stable and accurate indicator of pancreatic function. A normal or high C-peptide level generally indicates Type 2 diabetes (where the body produces insulin but is resistant to it), while a very low or zero C-peptide level points to Type 1 diabetes (where the pancreas has stopped producing insulin entirely).