A fasting C-peptide test is a valuable blood diagnostic assay used to measure the level of C-peptide in the bloodstream after a period of fasting. C-peptide is a byproduct created when proinsulin splits into insulin and C-peptide in equal amounts. Because it is secreted in tandem with endogenous insulin, measuring fasting C-peptide helps healthcare providers evaluate how much natural insulin the pancreas is actively producing. This test is essential for differentiating between type 1 and type 2 diabetes, assessing insulinoma tumors, and monitoring pancreatic function in patients managing diabetes treatment plans effectively.