C-Peptide Test: Normal Range

What Is C-Peptide?

C-peptide is released in equal amounts to insulin when your pancreas produces it. Measuring C-peptide shows how much insulin your body is actually making, even if you are taking insulin injections.

Normal C-Peptide Range

Test	Normal Range
Fasting C-peptide	0.5–2.0 ng/mL
Fasting Insulin (paired)	2–25 µIU/mL

Why It's Ordered

Distinguishing Type 1 from Type 2 diabetes
Evaluating unexplained low blood sugar (hypoglycaemia)
Assessing pancreatic insulin-producing capacity
Monitoring insulinoma (insulin-producing tumour)

What Low C-Peptide Means

Low levels suggest the pancreas is making little or no insulin: typical in Type 1 diabetes or advanced Type 2 diabetes.

What High C-Peptide Means

High levels suggest the pancreas is overproducing insulin, seen in early Type 2 diabetes, insulin resistance, or rarely an insulinoma.

Frequently Asked Questions

Can I take a C-peptide test if I'm already on insulin?

Yes. That's one of its main strengths. Because C-peptide reflects only your body's own insulin production (injected insulin contains no C-peptide), it can reveal how much natural insulin you still make even while on injections.

What does a high C-peptide level mean?

High C-peptide usually indicates your pancreas is producing large amounts of insulin, which is common in type 2 diabetes with insulin resistance, obesity, or, rarely, an insulin-producing tumour (insulinoma).

Do I need to fast before a C-peptide test?

Often yes: a fasting sample (8–12 hours) is standard, sometimes paired with a glucose reading. Your clinic will tell you if a stimulated (post-meal) test is needed instead.

How C-peptide is used in practice

The main clinical value of C-peptide is distinguishing between types of diabetes when the picture is unclear, for example in a younger adult who could have late-onset type 1 or early type 2 diabetes. A low C-peptide alongside high glucose indicates the pancreas is producing little insulin, supporting type 1; a normal or high result with high glucose suggests insulin resistance, supporting type 2.

It is also central to investigating unexplained low blood sugar. If someone has hypoglycaemia with a high C-peptide, their body is producing excess insulin, which raises the possibility of an insulin-producing tumour or the effect of certain diabetes tablets. If C-peptide is low while insulin is high, this suggests insulin has come from an injection rather than from the pancreas.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

Biochemistry, Hormones. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK541112

C-Peptide Test: Normal Range & What It Tells You