C‑Peptide is released in equal amounts to insulin when proinsulin is split into insulin and C‑peptide.

Unlike insulin, C‑peptide is not affected by exogenous insulin therapy, making it a reliable marker of pancreatic beta‑cell activity.

Clinical uses include:

Differentiating type 1 diabetes (low/absent C‑peptide) from type 2 diabetes (normal/high C‑peptide).

Assessing residual beta‑cell function in diabetic patients.

Evaluating insulinoma (tumor producing insulin).

Monitoring patients with pancreatic disease or post-pancreatectomy.

The test is performed using a blood sample, sometimes alongside glucose or insulin levels.

Analyte: Serum C‑peptide concentration (ng/mL).

Normal Range (varies by lab): ~0.5 – 2.0 ng/mL (fasting).

Abnormal Findings:

Low/undetectable C‑peptide: Suggests type 1 diabetes, pancreatic failure, or post-surgical removal.

High C‑peptide: Indicates type 2 diabetes, insulin resistance, or insulinoma.

Clinical Use:

Provides a direct measure of endogenous insulin secretion.

Helps guide diabetes classification and treatment decisions.

Useful in distinguishing hypoglycemia due to insulin therapy vs. insulinoma.