C-peptide Testing: Fasting Requirements
For a C-peptide test, an 8-hour fast is generally recommended, though a random non-fasting C-peptide sample taken within 5 hours of eating can also be used for diabetes classification. 1, 2
Fasting vs. Non-fasting C-peptide Testing
- An 8-hour fast is the standard recommendation for C-peptide testing according to clinical guidelines 1
- The American Diabetes Association recognizes that a random C-peptide sample collected within 5 hours of eating can replace a formal C-peptide stimulation test for diabetes classification purposes 2
- When C-peptide testing is required by insurance payers for coverage of insulin pump therapy, a fasting C-peptide level should be measured when the simultaneous fasting plasma glucose is ≤220 mg/dL 2
Clinical Significance and Interpretation
- C-peptide values <200 pmol/L are consistent with type 1 diabetes 2
- C-peptide values between 200-600 pmol/L may indicate type 1 diabetes, maturity-onset diabetes of the young (MODY), or insulin-treated type 2 diabetes 2
- C-peptide values >600 pmol/L suggest type 2 diabetes 2
- Random non-fasting C-peptide measurements have shown strong correlation with stimulated C-peptide levels from formal mixed meal tolerance tests, with high sensitivity and specificity for identifying clinically relevant thresholds 3
Important Testing Considerations
- C-peptide testing should not be performed within 2 weeks of a hyperglycemic emergency 2
- For insulin-treated patients, C-peptide should be measured prior to insulin discontinuation to exclude severe insulin deficiency 2
- When interpreting non-fasting C-peptide results, concurrent glucose levels should be considered - the correlation between random and stimulated C-peptide improves when excluding samples with concurrent glucose <8 mmol/L 3
- For neuroendocrine tumor evaluation, dietary adjustments may be necessary in addition to fasting requirements 1
Clinical Applications
- C-peptide testing helps distinguish between type 1 and type 2 diabetes in ambiguous cases 4, 5
- Low or undetectable levels of C-peptide indicate little or no insulin secretion, as seen in advanced type 1 diabetes 4
- C-peptide levels may predict clinical partial remission during the first year of type 1 diabetes 5
- Persistence of C-peptide is an important clinical feature of MODY (Maturity Onset Diabetes of the Young) 5
While fasting is generally recommended for optimal standardization of C-peptide testing, recent evidence suggests that random non-fasting measurements can provide clinically valuable information when interpreted appropriately with concurrent glucose levels.