A Practical Guide to C-Peptide Testing: How This Simple Blood Test Helps Diagnose and Manage Diabetes

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.

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.

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.

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.

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.