What this test measures
When pancreatic beta-cells produce insulin, they secrete it as proinsulin, which is cleaved into insulin and C-peptide in equimolar amounts. Unlike insulin, C-peptide is not removed by the liver — so it has a longer half-life and is a more reliable marker of endogenous insulin production. The serum assay measures fasting or stimulated C-peptide concentration.
C-peptide is unaffected by exogenous insulin (which contains no C-peptide), which makes it the ideal marker for distinguishing endogenous from exogenous insulin sources — central to the workup of unexplained hypoglycaemia.
Why it matters
C-peptide is the standard test to differentiate type 1 from type 2 diabetes when the type is unclear — particularly in adults presenting with new-onset diabetes that does not fit a typical pattern, and in children where Latent Autoimmune Diabetes (LADA) or MODY is being considered. A very low fasting C-peptide (< 0.6 ng/mL) with a high glucose strongly suggests type 1 or late-stage type 2 with beta-cell failure; a preserved or raised C-peptide suggests type 2.
In unexplained hypoglycaemia, C-peptide is the key discriminator: high insulin + high C-peptide = endogenous hyperinsulinism (insulinoma, sulfonylurea use); high insulin + low C-peptide = exogenous insulin (factitious or accidental). C-peptide is also part of the post-bariatric hypoglycaemia and Hirata-disease (insulin autoimmune syndrome) workups.
How to prepare
Fasting for 8–10 hours is the standard for a baseline measurement. For "stimulated" C-peptide (after a mixed meal or glucagon), follow your doctor's specific instructions. Stop biotin for 48–72 hours. Continue insulin and other diabetes medications as prescribed, but note timing on the requisition.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| C-Peptide (ng/mL)[1][2] | Fasting: 0.8 – 3.1 ng/mL · Stimulated (post-meal): 1.6 – 8.0 (assay-dependent) | Low fasting C-peptide (< 0.6) with high glucose = type 1 diabetes or late-stage type 2 with beta-cell exhaustion. Low C-peptide during documented hypoglycaemia with high insulin = exogenous insulin (factitious or accidental). | High fasting C-peptide with high glucose = type 2 diabetes with hyperinsulinaemia (insulin resistance). High C-peptide during documented hypoglycaemia with high insulin = endogenous hyperinsulinism — insulinoma, sulfonylurea use, or nesidioblastosis. Renal failure raises C-peptide because clearance is reduced. |
C-peptide — interpretation patterns
| Scenario | Glucose | Insulin | C-peptide | Interpretation |
|---|---|---|---|---|
| Adult with new diabetes | High | Low | Low (< 0.6) | Type 1 or LADA |
| Adult with new diabetes | High | High | High | Type 2 with insulin resistance |
| Hypoglycaemia, lean adult | Low | High | High | Endogenous hyperinsulinism (insulinoma, sulfonylurea) |
| Hypoglycaemia, healthcare worker | Low | High | Low | Exogenous insulin (factitious) |
| Renal failure | Variable | Variable | Falsely high | Reduced clearance |
Frequently asked questions
Why is C-peptide useful when we can just measure insulin?
C-peptide is co-secreted with insulin but is not removed by the liver, so it gives a more reliable picture of endogenous insulin production. Crucially, exogenous insulin contains no C-peptide — so C-peptide is the only way to separate "own" insulin from injected insulin.
How does C-peptide distinguish type 1 from type 2 diabetes?
Type 1 has progressive beta-cell destruction; C-peptide falls and is usually very low at presentation. Type 2 typically has preserved or high C-peptide (the beta-cells are intact, sometimes hyperactive). A low C-peptide with positive autoantibodies confirms type 1 or LADA.
I am on insulin — will the test work?
Yes. Because exogenous insulin contains no C-peptide, the test measures only your own residual insulin production — which is what your doctor needs to know.
What is a "stimulated" C-peptide test?
A mixed-meal or glucagon stimulation test, where C-peptide is measured before and after a standardised stimulus. The stimulated peak is more sensitive than fasting C-peptide for detecting residual beta-cell function, particularly in late type 2 diabetes.
Can renal failure affect C-peptide?
Yes — C-peptide is cleared by the kidneys, so chronic kidney disease can falsely raise the level. The trend over time is still informative; the absolute number needs caution.
How does this help with hypoglycaemia?
In any patient with unexplained hypoglycaemia, the question is "is the body making too much insulin, or has insulin been injected?" High C-peptide answers the first; low C-peptide with high insulin answers the second.
How often is C-peptide repeated?
Usually once for diagnosis, then occasionally in longitudinal monitoring of a known patient — for example, to document declining beta-cell function in late type 2 diabetes or LADA.
Related Diabetes / Glucose tests
Tests commonly ordered alongside C-PEPTIDE, or that help interpret an unexpected result.
Sources & references
- ADA Standards of Care in Diabetes 2025 · accessed 2026-05-30T00:00:00.000Z
- Endocrine Society — Hypoglycemia in Adults · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — C-Peptide Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — C-Peptide · accessed 2026-05-30T00:00:00.000Z
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