Normal C-peptide Levels in ng/ml
According to the American Diabetes Association's 2024 Standards of Care, normal C-peptide levels are considered to be above 1.8 ng/mL (>600 pmol/L), which is consistent with a type 2 diabetes pattern. 1, 2
Reference Ranges for C-peptide
The following C-peptide ranges can be used for clinical interpretation:
- <0.24 ng/mL (<80 pmol/L): Very low levels, consistent with severe insulin deficiency 1
- <0.6 ng/mL (<200 pmol/L): Type 1 diabetes pattern 2
- 0.6-1.8 ng/mL (200-600 pmol/L): Indeterminate status, may be seen in type 1 diabetes, MODY (maturity-onset diabetes of the young), or insulin-treated type 2 diabetes 1, 2
- >1.8 ng/mL (>600 pmol/L): Type 2 diabetes pattern or normal pancreatic beta cell function 1, 2
Clinical Context for C-peptide Testing
C-peptide is produced in equimolar amounts to endogenous insulin but has a longer half-life, making it a reliable marker of beta-cell function. Important considerations include:
- C-peptide testing is only indicated in people receiving insulin treatment 1
- A random C-peptide sample (with concurrent glucose) within 5 hours of eating can replace a formal C-peptide stimulation test for classification purposes 1
- For accurate assessment, C-peptide should be measured when fasting plasma glucose is ≤220 mg/dL (12.5 mmol/L) 2
- C-peptide testing is not recommended for routine screening for diabetes or cardiovascular disease risk 2
Interpretation in Different Clinical Scenarios
- Diabetes Classification: C-peptide helps distinguish between type 1 and type 2 diabetes when clinical presentation is ambiguous 2
- Insulinoma Diagnosis: In suspected insulinoma, C-peptide >3 mcIU/mL (usually >6 mcIU/mL) when blood glucose is <40-45 mg/dL, with an insulin-to-glucose ratio ≥0.3 indicates inappropriate insulin secretion 2
- Checkpoint Inhibitor-Associated Diabetes: Diagnosis includes new-onset hyperglycemia with low C-peptide (<0.4 nmol/L) indicating absolute insulin deficiency 1
Clinical Pitfalls to Avoid
- Do not test C-peptide within 2 weeks of a hyperglycemic emergency as results may be misleading 1
- When interpreting C-peptide in insulin-treated patients, remember that values between 0.6-1.8 ng/mL can occur in both type 1 diabetes and insulin-treated type 2 diabetes, particularly in people with normal/low BMI or after long disease duration 1
- In patients with insulin antibodies, C-peptide responses may be impaired, potentially leading to falsely low values 3
C-peptide measurement provides valuable information about endogenous insulin production and can guide treatment decisions, particularly when the diabetes classification is unclear or when evaluating residual beta-cell function.