From the Guidelines
C-peptide measurement does not necessarily require fasting, as a random non-fasting sample can provide valuable information about endogenous insulin production. According to the most recent guidelines, a C-peptide test can be performed on a random sample, with concurrent glucose measurement, within 5 hours of eating, replacing the need for a formal C-peptide stimulation test in certain contexts 1. However, if the test is being performed alongside glucose measurements or as part of a comprehensive diabetes evaluation, fasting for 8-12 hours beforehand may be recommended to establish baseline levels and improve result interpretation.
When considering the use of C-peptide measurements, it's essential to note that they are particularly useful in distinguishing between type 1 and type 2 diabetes, as well as in evaluating suspected insulinoma or factitious hypoglycemia 1. The guidelines suggest that C-peptide values between 200-600 pmol/L are usually consistent with type 1 diabetes or maturity-onset diabetes of the young, but may also occur in insulin-treated type 2 diabetes, particularly in people with normal or low BMI or after long duration 1.
In terms of specific protocols, the guidelines recommend that if a C-peptide test is indicated, it should be measured prior to insulin discontinuation to exclude severe insulin deficiency, and not within 2 weeks of a hyperglycemic emergency 1. Additionally, if the result is <600 pmol/L and the concurrent glucose is <4 mmol/L or the person may have been fasting, consider repeating the test 1.
Key points to consider when measuring C-peptide include:
- A random non-fasting sample can be used for routine C-peptide testing
- Fasting may be recommended in certain clinical scenarios, such as comprehensive diabetes evaluation
- C-peptide measurement is useful in distinguishing between type 1 and type 2 diabetes
- Specific protocols should be followed for C-peptide testing, including measurement prior to insulin discontinuation and avoidance of testing within 2 weeks of a hyperglycemic emergency 1.
From the Research
C-Peptide Measurement Requirements
- C-peptide measurement can be used to assess endogenous insulin secretion in patients with diabetes 2, 3, 4.
- The measurement of C-peptide can be helpful in differentiating between type 1 and type 2 diabetes, as well as in identifying patients with latent autoimmune diabetes in adults (LADA) or maturity-onset diabetes of the young (MODY) 2, 3.
- C-peptide levels can be measured in serum or urine, and modern assays have made C-peptide measurement more reliable and inexpensive 3, 4.
- High uncorrected fasting C-peptide in the presence of hyperglycemia may suggest insulin resistance 2.
Fasting Requirements for C-Peptide Measurement
- Some studies suggest that fasting C-peptide measurement can be useful in clinical practice, particularly in identifying patients with insulin resistance or type 2 diabetes 2, 3.
- However, other studies indicate that C-peptide measurement can be performed using spot urine samples or stimulated serum sampling, which may not require fasting 3, 4.
- The choice of C-peptide test and interpretation of results may depend on the clinical context and the specific goals of the measurement 3, 4.
Clinical Utility of C-Peptide Measurement
- C-peptide measurement can be used to guide diabetes diagnosis and management, including the classification of diabetes type and the selection of treatment strategies 2, 3, 4.
- C-peptide levels can also be used to monitor the progression of diabetes and the response to treatment, as well as to identify patients who may be at risk for microvascular and macrovascular complications 4.
- The standardization of C-peptide measurements is important to ensure accurate and comparable results, particularly in clinical practice and research studies 5.