C-Peptide
C-peptide is released along with insulin, so it shows how much insulin your own pancreas is making, even if you take insulin.
AI-verified against cited sources. Awaiting expert reviewAlso called C-pepLOINC 1986-9
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What it is
Your pancreas makes insulin from a larger molecule, and C-peptide is the piece that is cut off. It is released in equal amounts to insulin, but it lasts longer in the blood and is not affected by insulin you inject. That makes it a useful way to see how much insulin your own body is making. It is usually measured in a blood sample, often after fasting.
Why it matters
Clinicians use C-peptide to look into low blood sugar, to understand how much insulin a person with diabetes still makes, and sometimes to help sort out the type of diabetes when it is unclear. It is not used to diagnose diabetes, and it is read together with a blood sugar from the same time.
If it’s low
A low level means the pancreas is making little insulin. This is expected in type 1 diabetes, where it can be very low or undetectable, and it can happen in some people with longer-standing type 2 diabetes. Taking a lot of insulin can also hold your own production down.
Common causes
- Type 1 diabetes, and in some cases type 2 diabetes
- Taking too much insulin, which can switch off the pancreas's own production
- A severe infection
- Addison disease
- Liver disease
What can help
- Ask your clinician what the level means for your diabetes care plan
- Ask whether other tests, such as diabetes antibody tests, would help put the result in context
If it’s high
A high level usually means the body is making a lot of insulin. That is common when the body resists insulin, as in type 2 diabetes, and it also happens when the kidneys clear C-peptide more slowly. During an episode of low blood sugar, a high C-peptide can point to the body's own insulin as the cause, which is why clinicians read it with the glucose.
Common causes
- Type 2 diabetes and insulin resistance
- Reduced kidney function, because the kidneys clear C-peptide
- Eating shortly before the test
- Cushing's syndrome
- Certain diabetes medicines that raise the body's own insulin release
- An insulin-making tumor of the pancreas (uncommon)
What can help
- Ask your clinician how this result fits with your glucose and A1c
- If you have kidney problems, ask whether that could explain a high level
- Regular physical activity, a balanced diet and a healthy weight improve insulin sensitivity, and are worth discussing with your clinician
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
For adults: 0.8–3.9 ng/mL. Source: Quest Diagnostics, Labcorp, ARUP Laboratories, American Board of Internal Medicine.
- Adults (this one applies)0.8–3.9 ng/mL
Sources: Quest Diagnostics, Labcorp, ARUP Laboratories, American Board of Internal Medicine
What can affect your result
- Fasting or eating: levels rise after a meal
- Your blood glucose at the time of the draw
- Kidney function: the kidneys clear most C-peptide
- Insulin and diabetes medicines
- Body weight and insulin resistance
- How quickly the sample is separated and kept cold
- Not fasting: C-peptide rises after eating, so a non-fasting result will usually be higher than the fasting interval.
- Kidney disease slows the clearance of C-peptide and raises the result.
- C-peptide breaks down quickly in an unseparated sample. ARUP lists serum as stable for 8 hours at room temperature, 7 days refrigerated and 1 month frozen, and rejects grossly hemolyzed samples.
- Large amounts of antibodies against insulin can bind C-peptide and proinsulin and falsely raise the result on some assays.
Often a morning draw after an overnight fast (Quest asks for 12 hours, Labcorp 14 to 16 hours). Some tests are done after a meal on purpose, so follow your clinician's instructions. Changes within minutes to hours (its half-life is about 20 to 30 minutes), so it reflects the moment of the draw rather than a long-term average.
When to talk to a clinician
A C-peptide result on its own rarely needs urgent action. Talk with a clinician soon if you have repeated episodes of shakiness, sweating, fast heartbeat, dizziness or hunger that could be low blood sugar. If you are confused, cannot stay awake or have a seizure, call 911 or your local emergency number.
Questions you could ask
- Was this a fasting sample, and was a glucose measured at the same time?
- Which range is my lab comparing this with?
- How does this C-peptide fit with my glucose and A1c?
- Could my kidney function be part of why it is high?
- Would it help to repeat it after fasting?
- What does this level mean for how much insulin my body makes?
Questions people ask
Why is C-peptide read with a blood sugar?
C-peptide rises when blood sugar rises, because the pancreas releases more insulin to match. The same number can mean different things when your sugar is very low or very high, so clinicians read the two together.
Is C-peptide used to diagnose diabetes?
No. Blood glucose and A1c tests are used for that. C-peptide helps your clinician understand how much insulin your body makes, and it can help decide between type 1 and type 2 diabetes when the picture is unclear.
How do I convert C-peptide between ng/mL, nmol/L and pmol/L?
Multiply ng/mL by 0.331 to get nmol/L, or by 331 to get pmol/L. To go back, multiply nmol/L by 3.02. ARUP rounds the first factor to 0.33.
Show the science and sources
Units
- 1 ng/mL = 1 ng/mL
- 1 ng/mL = 0.3311 nmol/L
- 1 ng/mL = 331.1 pmol/L
Open the C-Peptide unit converter
Is a change real?
C-Peptide naturally varies about 16.6% from test to test in the same healthy person. Between two results from the same lab, a rise of 67% or more, or a fall of 40% or more, is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Desirable Biological Variation Database specifications, 2014 update (Minchinela J, Ricos C, et al.; the 'Ricos database', a compilation of median published CVI/CVG).
For clinicians and researchers
C-peptide (31 amino acids, about 3,020 g/mol) is cleaved from proinsulin and secreted in equimolar amounts with insulin. It has negligible hepatic extraction, a half-life of about 20 to 30 minutes (insulin 3 to 5 minutes) and is cleared mainly by the kidneys, so levels run higher when GFR is reduced. In healthy people the fasting concentration is about 0.3 to 0.6 nmol/L, rising to 1 to 3 nmol/L after a meal. Modern immunoassays need under 10% proinsulin cross-reactivity, and large amounts of anti-insulin antibodies can falsely raise results. In diabetes, a C-peptide under 0.2 nmol/L (about 0.6 ng/mL) is associated with type 1 diabetes. A random non-fasting value under 0.2 nmol/L correlated with stimulated C-peptide (r = 0.91) in insulin-treated people, but values above that need confirmation, and glucagon-stimulated or mixed-meal tests are more sensitive (Leighton 2017). The ADA notes that late in type 1 diabetes there is little or no insulin secretion, shown by low or undetectable plasma C-peptide. In hypoglycemia work-ups, the Endocrine Society guideline (2009) advises measuring plasma glucose, insulin, C-peptide, proinsulin and beta-hydroxybutyrate during an episode when endogenous hyperinsulinism is suspected. A 2025 review of non-diabetic hypoglycemia describes insulin-mediated hypoglycemia as glucose of 55 mg/dL or lower with insulin of 3 uU/mL or higher, C-peptide of 0.6 ng/mL or higher, proinsulin of 5 pmol/L or higher and beta-hydroxybutyrate of 2.7 mmol/L or lower. Reference intervals are assay specific (fasting: Quest 0.80 to 3.85 ng/mL; Labcorp 1.1 to 4.4; ARUP 0.5 to 3.3; ABIM 0.8 to 3.1). ARUP converts to nmol/L by multiplying ng/mL by 0.33. Stimulated and urine C-peptide are separate markers.
LOINC 1986-9 (primary), 14633-2, 35195-7, 42180-0. Specimen: serum-or-plasma.
Sources
- C-Peptide (test code 372), Quest Diagnostics
- C-Peptide (test 010108), Labcorp
- Serum, C-Peptide (test 0070103), ARUP Laboratories
- ABIM Laboratory Test Reference Ranges (January 2026), American Board of Internal Medicine (2026)
- C-Peptide Test, MedlinePlus, National Library of Medicine
- A Practical Review of C-Peptide Testing in Diabetes, Diabetes Therapy (2017)
- 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes-2026, American Diabetes Association (Diabetes Care) (2026)
- Evaluation and management of adult hypoglycemic disorders: an Endocrine Society Clinical Practice Guideline, Journal of Clinical Endocrinology & Metabolism (2009)
- Pancreatic Neuroendocrine Tumors: Diagnostic Pitfalls of Non-Diabetic Severe Hypoglycemia: Literature Review and Case Report, Diagnostics (Basel) (2025)
- Human C-peptide (PubChem compound 16157840), PubChem, National Library of Medicine
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.