C-Peptide, 24-Hour Urine

C-Peptide, 24-Hour Urine (urinary C-peptide; UCP) assesses endogenous insulin secretion to inform diabetes classification and management. In children, it supports discrimination between type 1 diabetes and MODY; in adults, it helps distinguish type 2 diabetes from LADA. It also assists with insulin dose titration during the honeymoon phase of type 1 diabetes and with determining absolute insulin deficiency in type 2 diabetes when a transition to insulin therapy is being considered. In the workup of recurrent hypoglycemia, urinary C-peptide aids in separating endogenous hyperinsulinism (for example, insulinoma) from exogenous insulin administration.

C-peptide is released from pancreatic beta cells in equimolar amounts with insulin and serves as a direct indicator of endogenous insulin production. Although commonly measured in blood, a timed 24-hour urinary C-peptide collection correlates with serum concentrations and provides a noninvasive approach that facilitates repeat testing and use in pediatrics. In children, urinary C-peptide helps separate autoimmune type 1 diabetes—which involves beta-cell destruction and results in absolute insulin deficiency—from MODY, a monogenic defect of insulin secretion that typically does not require insulin therapy. In adults, it assists in differentiating type 2 diabetes from LADA, an autoimmune form with later onset that generally progresses to insulin dependence. Serial measurements can document declining beta-cell function in type 2 diabetes and support timely initiation of insulin therapy. In patients with recurrent hypoglycemia, urinary C-peptide helps distinguish endogenous hyperinsulinism (eg, insulinoma) from exogenous insulin use; the latter suppresses endogenous insulin secretion and yields low C-peptide.