From the Guidelines
C-peptide should ideally be measured in a fasting state, though it can also be measured after a meal as part of a stimulation test. For routine clinical assessment, fasting C-peptide provides the most reliable baseline measurement, with patients typically fasting for 8-12 hours before the blood draw. This fasting state minimizes the influence of recent food intake on insulin and C-peptide secretion, allowing for more consistent and interpretable results. However, in some clinical scenarios, measuring C-peptide after a standardized meal or glucose challenge (postprandial or stimulated C-peptide) can provide valuable information about the body's insulin secretory capacity in response to rising blood glucose. This approach is particularly useful when evaluating residual beta cell function in diabetes patients. The difference between fasting and stimulated C-peptide levels can help clinicians assess the pancreas's ability to increase insulin production when needed. C-peptide is measured instead of insulin directly because it has a longer half-life in the bloodstream and provides a more stable indicator of endogenous insulin production, especially in patients taking exogenous insulin. According to the most recent guidelines, a C-peptide test is only indicated in people receiving insulin treatment 1.
Some key points to consider when measuring C-peptide include:
- Fasting C-peptide level should be measured when simultaneous fasting plasma glucose is ≤220 mg/dL (12.5 mmol/L) 1
- A random sample (with concurrent glucose) within 5 hours of eating can replace a formal C-peptide stimulation test in the context of classification 1
- If the result is >600 pmol/L (>1.8 ng/mL), the circumstances of testing do not matter, but if the result is <600 pmol/L (<1.8 ng/mL) and the concurrent glucose is <4 mmol/L (<70 mg/dL) or the person may have been fasting, consider repeating the test 1
- Results showing very low levels (e.g., <80 pmol/L [<0.24 ng/mL]) do not need to be repeated 1
It's also important to note that C-peptide measurements may help distinguish type 1 from type 2 diabetes in ambiguous cases 1. However, the decision to measure C-peptide should be made on a case-by-case basis, taking into account the individual patient's clinical presentation and medical history.
From the Research
C-Peptide Measurement
- C-peptide can be measured in both fasting and non-fasting states to assess pancreatic beta cell function 2, 3, 4, 5, 6.
- The choice of measurement state may depend on the specific clinical context and the information being sought.
Fasting vs. Postprandial C-Peptide
- Fasting C-peptide levels can provide information on basal beta cell function, while postprandial C-peptide levels can reflect the beta cell response to a meal stimulus 2, 3, 6.
- The postprandial C-peptide to glucose ratio may be a useful marker of beta cell function, particularly in patients with type 2 diabetes 2.
Clinical Utility of C-Peptide Measurement
- C-peptide measurement can be used to assess residual beta cell function in patients with diabetes, which can inform treatment decisions and predict response to therapy 4, 5.
- Random non-fasting C-peptide measurement may be a practical and robust alternative to standard stimulation tests, allowing for assessment of endogenous insulin secretion in a clinical setting 5.
Specific Considerations
- In patients with long-standing type 1 diabetes, stimulated C-peptide may be detectable in some individuals, even if fasting C-peptide is not 6.
- The choice of measurement state and timing may depend on the specific clinical question being addressed, such as assessing beta cell function or predicting response to therapy 2, 3, 4, 5, 6.