C-peptide Testing Guidelines

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 at a certain threshold 2

C-peptide values less than a certain level are consistent with type 1 diabetes 2
C-peptide values between certain levels may indicate type 1 diabetes, maturity-onset diabetes of the young (MODY), or insulin-treated type 2 diabetes 2
C-peptide values above a certain level suggest type 2 diabetes 2

C-peptide testing should not be performed within a certain time period of a hyperglycemic emergency 2
For insulin-treated patients, C-peptide should be measured prior to insulin discontinuation to exclude severe insulin deficiency 2
For neuroendocrine tumor evaluation, dietary adjustments may be necessary in addition to fasting requirements 1

C-peptide testing helps distinguish between type 1 and type 2 diabetes in ambiguous cases 3
Low or undetectable levels of C-peptide indicate little or no insulin secretion, as seen in advanced type 1 diabetes 3