The C-peptide postprandial (PP) test measures the level of C-peptide in the blood typically two hours after a meal. C-peptide is a byproduct released by the pancreas in equal amounts alongside endogenous insulin, serving as a reliable indicator of beta-cell function. Because exogenous insulin medications do not affect C-peptide levels, this test offers a clear, stable assessment of how effectively the pancreas responds to food intake. Clinicians use the C-peptide PP test to help differentiate between type 1 and type 2 diabetes, evaluate insulin secretory capacity, and investigate causes of abnormal blood glucose regulation.