A C-Peptide Fasting Test (CPEP) measures C-peptide in your blood after fasting (usually 8-10 hours) to check your pancreas’s insulin production, helping doctors tell diabetes types apart (Type 1 vs. Type 2), monitor treatment, and assess pancreatic function, as C-peptide is released equally with insulin but lasts longer in the blood, making it a reliable indicator of your body’s natural insulin-making ability