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C-peptide
A byproduct of the body's own insulin production. Shows how much insulin the pancreas makes and helps tell diabetes types apart.
Other names: C-peptide, Connecting PeptideSection: Blood chemistry
Updated: August 31, 2026·MedProtect editorial team Reference information — not a diagnosis or a prescription
In shortC-peptide shows how much of its own insulin the pancreas produces. A typical fasting range is 0.3–3.3 ng/mL (0.2–1.0 nmol/L). Low values occur in type 1 diabetes, high ones in insulin resistance and type 2 diabetes; it must be read together with glucose.
Reference ranges
Group
Reference range
Fasting
0.3–3.3 ng/mL (0.2–1.0 nmol/L)
The source gives 0.3–3.3 ng/mL, corresponding to 0.2–1.0 nmol/L per the same source. The result is read as a pair with glucose: a 'normal' C-peptide means different things at high and at low blood sugar. Reference ranges vary between laboratories and assays — use the range printed on your own lab report.
Check your value
Enter the number from your lab report — see how it compares with the typical range. Mind the units shown in the chosen row.
A rough comparison with a typical range, not a diagnosis. Reference ranges vary between laboratories — check the range on your own report and let a clinician interpret the result.
What this marker is
C-peptide is a fragment split off the proinsulin molecule when active insulin is formed. The pancreas releases C-peptide and insulin in equal amounts, but C-peptide lives longer in blood and is not broken down by the liver, so it reflects the body's own insulin secretion more accurately.
Its main use is distinguishing diabetes types: in type 1, C-peptide is low or undetectable (almost no own insulin); in type 2, normal or raised. The test also helps find the cause of hypoglycemia: high C-peptide with low glucose points to excess insulin production (for example, an insulinoma), while low C-peptide points to injected insulin.
When it is high
Insulin resistance and type 2 diabetesThe pancreas compensates for tissue resistance by producing more insulin.
InsulinomaA beta-cell tumor releases excess insulin — high C-peptide against hypoglycemia.
Kidney failureC-peptide is excreted by the kidneys — its level rises in kidney disease.
ObesityExcess body mass raises the demand for insulin and its secretion.
When it is low
Type 1 diabetesBeta cells are destroyed by an autoimmune process — own insulin and C-peptide are barely produced.
Late-stage type 2 diabetesBeta-cell exhaustion after many years of disease lowers secretion.
Injected insulinIn hypoglycemia, low C-peptide means the insulin in blood is injected, not self-made.
Pancreatitis and pancreatic surgeryLoss of functioning gland tissue.
What can skew the result
These factors can shift the value without any disease — worth considering before a retest:
Food (test fasting)Insulin injections (do not affect it — the test's key value)Kidney diseaseObesity
How to prepare
Blood from a vein strictly fasting (8–12 hours without food). Sometimes the doctor orders a stimulated test — after a meal or glucagon injection. Always read the result together with a glucose level drawn at the same time: C-peptide without glucose cannot be interpreted.
When a retest makes sense
As the endocrinologist advises — when clarifying the diabetes type, changing therapy or investigating hypoglycemia episodes.
Who usually reviews this result
A result outside the reference range is usually reviewed by: endocrinologist. Bring your lab report and any previous results to the appointment.
Names used on lab reports
Your report may use any of these names: C-peptideConnecting peptideInsulin C-peptide
Units and conversion
Measured in: ng/mL
ng/mL=nmol/L
nmol/L = ng/mL × 0.331 (C-peptide, molar mass ≈3020)
nmol/L = ng/mL × 0.331 (approximate; rounding may differ slightly from the source).
Several markers on your report?
See how MedProtect explains results in the app: the whole report, a check of the recognized values, and a clear breakdown.
Insulin is quickly broken down by the liver and fluctuates, while C-peptide is more stable and shows own insulin output more precisely. Insulin injections do not affect C-peptide — unlike an insulin assay.
Why test C-peptide when diabetes is already known?
To see whether own secretion remains: this guides therapy choice (tablets or insulin) and helps clarify the diabetes type in ambiguous cases.
Should C-peptide be taken with glucose?
Yes, always as a pair: the same C-peptide level can be insufficient at high blood sugar and excessive at normal sugar.
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