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

Often yes, or it may be measured with a blood sugar result or after a stimulus; follow your lab's instructions.

What does a C-peptide test measure?

How much insulin your own pancreas is producing.

What does low C-peptide mean?

Usually low insulin production, as in type 1 or long-standing type 2 diabetes.

Is C-peptide used as a treatment?

Not as an approved drug; it has been studied for diabetic nerve damage without approval.

C-Peptide: What the Test Shows About Insulin

C-peptide explained: the peptide released with insulin, what a C-peptide test measures, and what high or low results can mean. Not medical advice.

Anonymous user report. Not medical advice. Peptides discussed are not FDA-approved for the uses described. Editorial process →

Short answer

C-peptide (connecting peptide) is a 31-amino-acid fragment that the pancreas releases in equal amounts with insulin when it splits proinsulin. Because it lasts longer in the blood than insulin and is not present in injected insulin, a C-peptide blood or urine test shows how much insulin your own pancreas is making. Doctors use it to help tell type 1 from type 2 diabetes, check remaining insulin production, and investigate unexplained low blood sugar.

What results can mean

Low C-peptide suggests the pancreas makes little insulin, as in type 1 diabetes or long-standing type 2. High C-peptide can occur with insulin resistance, early type 2 diabetes, kidney disease (which slows clearance) or, rarely, an insulin-producing tumour. If low blood sugar comes with high insulin but low C-peptide, that points to injected insulin rather than the body's own. Reference ranges vary by lab and by whether you fasted, so results need interpretation by a clinician. Last reviewed: October 2026.

How to read C-peptide results

General patterns; your lab's reference range and your clinician's interpretation take priority. Last reviewed October 15, 2026.

Pattern	Can suggest
Low C-peptide, high blood sugar	Type 1 diabetes or advanced loss of insulin production
Normal or high C-peptide, high blood sugar	Type 2 diabetes with insulin resistance
High C-peptide, low blood sugar, high insulin	Insulinoma or sulfonylurea use; needs specialist work-up
Low C-peptide, low blood sugar, high insulin	Insulin from injections rather than the pancreas
High C-peptide with kidney disease	Slower clearance, not necessarily more insulin

C-peptide and GLP-1 drugs

GLP-1 drugs such as semaglutide increase insulin release when blood sugar is high, so C-peptide can rise on treatment, but the test is not used to monitor them. Clinicians may check C-peptide before choosing between insulin and other diabetes drugs. See what is GLP-1.

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Community Q&A

Do I need to fast for a C-peptide test?: Often yes, or it may be measured with a blood sugar result or after a stimulus; follow your lab's instructions.
What does a C-peptide test measure?: How much insulin your own pancreas is producing.
What does low C-peptide mean?: Usually low insulin production, as in type 1 or long-standing type 2 diabetes.
What does high C-peptide mean?: It can reflect insulin resistance, early type 2 diabetes, kidney disease or, rarely, an insulinoma.
Is C-peptide used as a treatment?: Not as an approved drug; it has been studied for diabetic nerve damage without approval.

Anonymous first-hand reports plus sourced context — educational only, not medical advice. Editorial policy & corrections.