C-Peptide
The C-peptide test measures a stable cleavage product secreted equimolarly with insulin, reflecting endogenous beta-cell function independent of exogenous insulin and hepatic clearance. It classifies diabetes type, investigates hypoglycaemia, assesses residual insulin secretion, detects factitious insulin use, and monitors transplant graft function.
1
Biomarkers
Blood (serum)
Sample
2–4 business days
Turnaround
Yes (typically 8–12 hours for basal measurement; non-fasting or stimulated samples may be ordered depending on indication)
Fasting
What is the C-Peptide test?
The C-Peptide test quantifies a stable cleavage product secreted in equimolar amounts with endogenous insulin, offering a direct window into pancreatic beta-cell function that is not confounded by exogenous insulin or hepatic metabolism. Practitioners order it when the type or cause of diabetes is unclear, when hypoglycaemia of uncertain aetiology requires investigation, and whenever distinguishing endogenous from exogenous insulin matters to diagnosis or management. C-peptide is central to resolving diagnostic ambiguity: a patient with diabetes and preserved or elevated C-peptide has ongoing beta-cell secretion, pointing to type 2 diabetes, maturity-onset diabetes of the young, or another insulin-resistant or monogenic form, whereas low or undetectable C-peptide confirms absolute insulin deficiency typical of type 1 diabetes or late pancreatic failure. In hypoglycaemia, an elevated C-peptide during symptomatic hypoglycaemia indicates endogenous hyperinsulinism — insulinoma, nesidioblastosis, or sulphonylurea use — while a suppressed value in the presence of exogenous insulin suggests factitious administration. The test is also used to assess residual beta-cell function in established diabetes, informing decisions about insulin intensification, and to monitor graft function after islet or pancreas transplantation.
What does the C-Peptide test measure?
C-peptide is a 31-amino-acid cleavage product released in equimolar amounts with insulin when proinsulin is processed in pancreatic beta cells. Unlike insulin, it undergoes minimal hepatic first-pass extraction and has a longer plasma half-life of roughly 30 minutes, making it a more stable and direct indicator of endogenous insulin secretion. Because exogenous insulin preparations lack C-peptide, the assay distinguishes native beta-cell output from administered insulin.
C-peptide is the gold-standard marker of endogenous insulin production. It resolves the critical question of whether hyperinsulinaemia is endogenous or exogenous, differentiates type 1 from type 2 diabetes and monogenic forms, guides insulin therapy decisions, investigates suspected insulinoma, and assesses beta-cell reserve in established diabetes. Its stability and independence from hepatic clearance make it superior to insulin measurement in many contexts.
Why is the C-Peptide test ordered?
- Classification of diabetes when clinical presentation is atypical or when type 1 versus type 2 distinction is uncertain, especially in lean adults or children with apparent type 2 features
- Investigation of hypoglycaemia, particularly to distinguish endogenous hyperinsulinism (insulinoma, nesidioblastosis) from exogenous insulin or sulphonylurea administration
- Assessment of residual beta-cell function in patients with established diabetes, to guide decisions on insulin therapy intensification or pump candidacy
- Suspected or confirmed maturity-onset diabetes of the young (MODY) or other monogenic diabetes, where preserved C-peptide despite hyperglycaemia is a clue
- Evaluation of patients on insulin therapy in whom factitious hypoglycaemia (covert exogenous insulin) is suspected — low C-peptide with high insulin confirms exogenous source
- Monitoring of pancreatic or islet transplant recipients, where rising C-peptide signals graft function and falling levels indicate rejection or failure
- Investigation of insulin resistance syndromes, where elevated fasting C-peptide reflects compensatory beta-cell hypersecretion
- Pre-operative work-up of suspected neuroendocrine tumours when insulinoma is in the differential, often paired with a supervised fast
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
Yes (typically 8–12 hours for basal measurement; non-fasting or stimulated samples may be ordered depending on indication)
Collection method
Venipuncture at patient service centre
Turnaround
2–4 business days
Collection notes
Phlebotomy at the processing laboratory's draw site (LabCorp, Quest, Access Medical Laboratories, or Boston Heart Diagnostics, depending on routing). For fasting basal C-peptide, an overnight fast of 8–12 hours is standard; for insulinoma investigation or mixed-meal stimulation, follow the specific protocol (often a supervised 72-hour fast or a standardised mixed meal with serial sampling). Record concurrent glucose and, if applicable, insulin. Note the performing laboratory for serial comparability.
Specimen requirements
Minimum 0.5 mL serum (gel separator acceptable). Separate from cells and freeze if not assayed promptly; C-peptide is stable frozen for months. Document renal function and any secretagogue or insulin therapy, as these alter interpretation.
What can affect C-Peptide results?
C-peptide is cleared by the kidney; reduced glomerular filtration prolongs its half-life and raises circulating levels, so interpretation must account for renal function.
C-peptide rises postprandially and with glucose stimulation; fasting samples reflect basal secretion, while stimulated or random samples are higher and require context for interpretation.
Exogenous insulin suppresses endogenous secretion via negative feedback, lowering C-peptide in proportion to the degree of beta-cell suppression; this suppression is the basis for detecting factitious insulin use.
These secretagogues stimulate endogenous insulin and C-peptide release; awareness of medication history is essential when interpreting elevated C-peptide in hypoglycaemia.
Compensatory hyperinsulinaemia in insulin-resistant states raises both insulin and C-peptide; an elevated fasting C-peptide in this context reflects beta-cell adaptation rather than pathology.
C-peptide assays differ in calibration and reference ranges; serial measurements should use the same laboratory and method for comparability.
What do C-Peptide results mean?
Elevated
Elevated fasting C-peptide indicates endogenous hyperinsulinism — commonly insulin resistance in metabolic syndrome or type 2 diabetes, insulinoma or nesidioblastosis if hypoglycaemia is present, or sulphonylurea/meglitinide use. In renal impairment, elevated C-peptide may reflect reduced clearance rather than hypersecretion.
Normal
Normal fasting C-peptide in a patient with diabetes suggests preserved beta-cell function and points to type 2, early monogenic diabetes, or another non-autoimmune form. In hypoglycaemia, a normal or low C-peptide rules out endogenous hyperinsulinism.
Reduced
Low or undetectable C-peptide, especially fasting or after a mixed-meal tolerance test, confirms insulin deficiency typical of type 1 diabetes, late type 2 with beta-cell exhaustion, or pancreatectomy. Suppressed C-peptide with elevated insulin during hypoglycaemia strongly suggests exogenous insulin administration (factitious or iatrogenic).
Note: C-peptide must be interpreted with concurrent glucose, insulin (if measured), renal function, and medication history. Stimulated testing (mixed-meal tolerance test or glucagon stimulation) is often needed to demonstrate residual secretion or to provoke hypoglycaemia for insulinoma work-up. Reference intervals and units differ by assay; verify the laboratory's range.
Frequently asked questions
C-peptide distinguishes type 1 from type 2 diabetes: low or undetectable C-peptide indicates absolute insulin deficiency (type 1 or late pancreatic failure), while preserved or elevated levels suggest residual beta-cell function typical of type 2, monogenic, or other insulin-resistant forms.
How to access the C-Peptide test
The C-Peptide reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: October 7, 2026
Last reviewed: October 7, 2026
- American Diabetes Association. Classification and diagnosis of diabetes: Standards of Care in Diabetes. (2025)
- Cryer PE, et al. Evaluation and management of adult hypoglycemic disorders: an Endocrine Society clinical practice guideline. (2009)
- Jensen MV, et al. Circulating C-peptide as a biomarker of beta-cell function in diabetes: a review. Diabetologia. (2021)
- Service FJ, et al. Diagnostic approach to hypoglycaemia in adults. (Landmark reference, verify details). (2012)
- Palmer JP, et al. C-peptide in the natural history of type 1 diabetes. Diabetes Metab Res Rev. (2009)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.