C-peptide testing — especially using the glucagon stimulation method — is a reliable way to measure how much insulin the pancreas is still producing, helping doctors distinguish diabetes types and guide treatment.
< 0.2 nmol/LC-peptide level strongly associated with type 1 diabetes diagnosis
What the researchers found
C-peptide is produced in equal amounts to insulin but is excreted more steadily, making it a reliable marker of how much insulin the pancreas is actually producing. A C-peptide level below 0.2 nmol/L is strongly associated with a diagnosis of type 1 diabetes. The authors recommend glucagon stimulation testing as the best balance of sensitivity and practicality among available methods. C-peptide levels also correlate with microvascular and macrovascular complications, future need for insulin therapy, and likely response to individual treatments.
Why it matters
Many people with diabetes receive an uncertain diagnosis — especially those who develop diabetes later in life but may actually have type 1 or latent autoimmune diabetes. C-peptide testing can clarify what type of diabetes someone has, predict complications, and guide treatment decisions. This review consolidates the evidence into practical recommendations clinicians can use.
How the study worked
The authors conducted a narrative review of the published literature on C-peptide testing methods, including urinary C-peptide, unstimulated serum C-peptide, and stimulated serum C-peptide (via glucagon stimulation test). They evaluated the sensitivity, practicality, and clinical utility of each approach.
What this study cannot tell us
As a narrative review rather than a systematic review or meta-analysis, the paper does not quantitatively pool data from multiple studies. The specific cutoff values cited may vary depending on the assay used and the population studied. The review focuses primarily on clinical utility rather than exploring emerging research applications of C-peptide measurement.
How to read the evidence
This is a narrative review that synthesizes existing clinical evidence on C-peptide testing. It provides practical recommendations but does not generate new primary data or perform a systematic analysis.
When this study was published
Published in 2017, this review remains clinically relevant as C-peptide testing methods and cutoff values have not changed substantially. The core recommendations still reflect current practice.
The bigger picture
C-peptide sits at the intersection of diagnostics and treatment planning in diabetes. As the peptide therapeutics landscape expands — with GLP-1 agonists, amylin analogs, and insulin innovations — knowing exactly how much natural insulin a patient still produces becomes increasingly important for personalizing therapy. C-peptide testing is one of the oldest and most established clinical applications of peptide measurement.
Questions still open
- Could continuous or repeated C-peptide monitoring help optimize the timing of insulin initiation in type 2 diabetes?
- How do C-peptide levels interact with newer peptide therapies like GLP-1 agonists in guiding combination treatment?
- Should C-peptide testing be used more routinely in adults diagnosed with diabetes after age 30 to screen for latent autoimmune diabetes?
Common questions
What is C-peptide and why is it used instead of measuring insulin directly?
What does a low C-peptide level mean?
Read the original research
A Practical Review of C-Peptide Testing in Diabetes.
Diabetes therapy : research, treatment and education of diabetes and related disorders, 8(3), 475-487
Citation
Leighton, Emma; Sainsbury, Christopher Ar; Jones, Gregory C. (2017). A Practical Review of C-Peptide Testing in Diabetes.. Diabetes therapy : research, treatment and education of diabetes and related disorders, 8(3), 475-487. https://doi.org/10.1007/s13300-017-0265-4