C-Peptide
A by-product released with insulin, used to gauge how much insulin the pancreas is making.
| Typical reference range | 0.8–3.85 ng/mL |
Ranges vary by laboratory, method, age and sex: always use the range printed on your own report. Why labs differ.
What it measures
When the pancreas makes insulin, it cleaves proinsulin into insulin and C-peptide in equal amounts. Unlike insulin, C-peptide is not substantially cleared by the liver, so it reflects pancreatic insulin output more stably.
Why it matters
C-peptide distinguishes the body's own insulin production from injected insulin and helps classify diabetes (low in type 1, normal or high early in type 2). Elevated fasting C-peptide indicates insulin resistance with compensatory hypersecretion.
What a high result can mean
High values occur with insulin resistance, obesity, early type 2 diabetes, kidney disease (reduced clearance), sulfonylurea use and, rarely, insulinoma.
What a low result can mean
Low values suggest reduced beta-cell function, as in type 1 diabetes, longstanding type 2 diabetes or pancreatic damage; also seen with low glucose from injected insulin.
What affects the result
Usually measured fasting together with glucose. Kidney function strongly affects levels because C-peptide is cleared by the kidneys.
Units and conversion
Usually reported in ng/mL.
- From nmol/L: multiply by 3.02 to get ng/mL
- From pmol/L: multiply by 0.003 to get ng/mL
Also called on lab reports
C Peptide, Connecting Peptide, C-Peptide, Serum
Related tests: Glucose & Metabolic
This page is educational and does not replace advice from a healthcare professional.