What is C-peptide?
C-peptide is a piece of protein your pancreas makes together with insulin. The beta cells first build one long protein, proinsulin, and cut a connecting piece out of it just before release. What remains is insulin; the piece cut out is C-peptide, the C for connecting. For every insulin molecule exactly one C-peptide is released. That makes C-peptide a counter of your own insulin production, and on two points a better one than insulin itself. First, it is absent from insulin out of a pen: anyone who injects insulin has an insulin value that comes from outside, but a C-peptide that can only come from their own pancreas. Second, it stays in your blood longer. Insulin is largely captured by your liver before it reaches the rest of your body and is gone within minutes; C-peptide is cleared by your kidneys and stays measurable for half an hour or more, which makes it fluctuate less. Your report shows C-peptide in nmol/L, measured fasting. There is no national threshold and the range differs per lab, so compare your value with your own lab's table, and always read it beside the fasting glucose from the same draw.
Why is C-peptide relevant?
The question C-peptide answers is: how much insulin do you still make yourself? For anyone healthy with a normal blood sugar the answer is almost always enough, and then the test adds little to a fasting insulin. To be honest: this is not a value everyone needs to know every year. It becomes important once blood sugar is no longer normal, because then there are two very different explanations that call for opposite approaches. In type 2 diabetes the cells listen poorly and the pancreas makes plenty: a high C-peptide. In type 1 and in LADA, the slow form of type 1 that develops in adulthood, the immune system breaks down the beta cells and production of your own falls away: a low C-peptide. The two look alike from the outside, and the distinction is missed more often than you would think. Of people diagnosed with type 2, an estimated 15 percent actually have LADA and 1 to 5 percent MODY, an inherited form. For anyone with type 2, C-peptide also offers perspective. A pancreas still producing plenty is a pancreas lifestyle can still do a lot with: losing weight and exercising make the cells sensitive again, and your own insulin then does the rest. A low production of your own says the beta cells are exhausted or under attack, and then insulin from outside is the way. C-peptide is the number that keeps those two apart. Where LADA is suspected, the distinction is confirmed with antibodies, not with C-peptide alone. And for anyone who injects insulin, C-peptide is the only way to see what the pancreas itself still does, because an insulin value then says nothing any more.
What Dutch labs actually use
Two laboratories, the same unit, and still a different range at both ends. Unilabs publishes 0.26 to 1.39 nmol/l, Clinical Diagnostics 0.16 to 1.10 nmol/l. Someone whose fasting value lands on 1.2 sits inside one laboratory's range and above the other's, with nothing about the blood having changed. We could not find Star-shl's table under three plausible addresses, which is not evidence that they do not run it, only that we could not locate it.
0.26 – 1.10nmol/L
Between 0.16 and 0.26 and between 1.10 and 1.39 it depends on the lab, because each sets its limits on its own method and population. Above 1.39 every lab calls it abnormal.
every lab calls 0.26 – 1.10 nmol/L normal. Between 0.16 and 0.26 and between 1.10 and 1.39 it depends on the lab.
Every age, per lab
| Laboratory | Group | Reference range |
|---|---|---|
| Unilabs(Saltro, Medlon, SHO, Atalmedial)2026 | Nuchter | 0,26–1,39 nmol/L |
| Clinical Diagnostics2026 | Nuchter | 0,16–1,10 nmol/L |
Each band as the lab publishes it, retrieved in 2026, unless marked otherwise.
C-peptide high or low: what it means
A high C-peptide means a pancreas making a lot of insulin, and the usual reason is the same as for a high insulin: insulin resistance from belly fat, little exercise, little muscle, plenty of sugar and lack of sleep. Anyone not drawn fasting measures production as a response to that meal, and that sits several times higher. Medication that stimulates release, certain diabetes medicines, lifts the value too. And because your kidneys clear C-peptide, the value rises with reduced kidney function without production changing. A low C-peptide with a normal glucose is, in a healthy person, usually a sensitive metabolism that gets by on little insulin, and so favourable. Low beside a high glucose is the picture of a pancreas that can no longer keep up: exhausted after years of type 2, under attack in type 1 and LADA, or built with a fault in the beta cell as in MODY. A long fast also lowers the value, because without glucose coming in little insulin is needed. With a high C-peptide the levers are those of insulin resistance, and they work: losing weight around the middle, strength training and daily movement, less sugar and fast carbohydrates, more fibre and protein, enough sleep. As your cells become more sensitive, the pancreas has to make less and the number falls with it. A low C-peptide with a high glucose cannot be raised with lifestyle; there production of your own is gone, and insulin from outside is the designated route. In practice C-peptide is a fragile test: the tube has to be processed or frozen within a few hours, so you cannot have it drawn at every location, and a failed measurement means a new draw. Read the value beside the fasting glucose from the same draw, and beside HbA1c for the longer story. Anyone without diabetes who only wants to know how sensitive their metabolism is usually has enough with fasting insulin or the TyG index.
What lowers it
A sensitive metabolism, or a pancreas releasing less; the glucose beside it makes the difference.
| Cause | How often |
|---|---|
| Healthy insulin sensitivityThe ordinary outcome in anyone lean and fit: your cells listen well and your pancreas needs to make little, so little C-peptide is released either | Often |
| Long-standing type 2 diabetesAfter years of overwork the beta cells become exhausted and production of your own falls, while the glucose stays high | Often |
| Type 1 diabetes and LADAThe immune system breaks down the beta cells; in LADA that happens slowly and in adulthood, and at first it looks like type 2 | Sometimes |
| A beta-cell defect such as MODYAn inherited fault in the beta cell that makes the production and release of insulin falter from a young age | Rare |
What raises it
A pancreas working hard, and beyond that the draw and the kidneys.
| Cause | How often |
|---|---|
| Insulin resistanceBelly fat, little exercise, plenty of sugar and lack of sleep make your cells less sensitive; your pancreas makes more insulin and so more C-peptide too | Often |
| Not drawn fastingAfter a meal production rises several times over; anyone who has eaten measures that response and not the starting point | Often |
| Medication that stimulates releaseCertain diabetes medicines, the sulfonylureas, set the beta cells to releasing more | Sometimes |
| Reduced kidney functionYour kidneys clear C-peptide; if they work less well it stays in your blood longer and the value rises without production changing | Sometimes |
How does a C-peptide blood test work?
- Referral
- Not needed. You can have C-peptide measured without going through your GP.
- Fasting
- Yes: at least 8 hours without food and preferably 12, water only. Anyone who has eaten measures production as a response to that meal and not the starting point.
- Where you can go
- Not everywhere. The tube has to be processed or frozen within a few hours, so only locations that can do that take it, often a hospital outpatient clinic. Check before you schedule an appointment.
- The draw
- One tube of blood from a vein in your arm. The blood is spun quickly and the serum sent to the lab chilled or frozen.
- One chance per tube
- A failed measurement cannot be repeated later from the same tube; if something goes wrong on the way, a new draw follows.
- The result
- A single number in nmol/L. Count on about two weeks, because the test does not run daily at every location.
- Where to draw
- You can draw at 250+ locations near you, no referral needed. See the locations
Frequently asked questions
What is C-peptide?
C-peptide is the piece of protein left over when your pancreas makes insulin. For every insulin molecule one C-peptide is released, so it counts how much you produce yourself. Unlike insulin it is not immediately captured by your liver, which makes it more stable to measure. And it is absent from insulin out of a pen, so it distinguishes your own insulin from injected insulin.
What is a normal C-peptide level?
There is no national threshold, and the range differs per lab: one uses 0.26 to 1.39 nmol/L fasting, another 0.16 to 1.10 nmol/L. So use your own lab's range. More important than the range is the glucose beside it: a value inside the range with a normal blood sugar is a pancreas simply coping, the same value with a high blood sugar is a pancreas no longer keeping up.
What does a low C-peptide mean?
Beside a normal glucose usually a sensitive metabolism that gets by on little insulin, and that is favourable. Beside a high glucose it means your pancreas makes little insulin of its own any more: exhausted after years of type 2 diabetes, attacked by the immune system in type 1 and LADA, or with an inherited fault in the beta cell as in MODY. Then insulin from outside is the way, and lifestyle helps with the rest but not with the production.
What does a high C-peptide mean?
That your pancreas makes a lot of insulin, and the usual reason is insulin resistance: cells that, from belly fat, little exercise, plenty of sugar and lack of sleep, listen less well to insulin, so more of it is needed. That is also good news, because a pancreas still producing plenty is a pancreas losing weight and exercising can do a lot with. With a high value, first check whether you were fasting, and bear in mind that reduced kidney function lifts the value without production changing.
What does C-peptide add to an insulin test?
Two things. The distinction between production of your own and injected insulin: both are released at the same cut, but C-peptide is absent from insulin out of a pen, so an insulin value can come from outside and a C-peptide cannot. And stability: insulin is captured by your liver within minutes, C-peptide stays measurable for half an hour or more and therefore fluctuates less.
Do I need C-peptide if I do not have diabetes?
Usually not. With a normal blood sugar your pancreas makes enough, and the question of how sensitive your metabolism is gets answered just as well by a fasting insulin or the TyG index, without the strict draw conditions. C-peptide only becomes truly valuable once blood sugar is no longer normal and the question is whether the pancreas is overworking or becoming exhausted.
Is LADA diagnosed with C-peptide?
Not with C-peptide alone. LADA, the slow form of type 1 that develops in adulthood, is confirmed with antibodies against the beta cells, usually anti-GAD65; a low C-peptide with a high glucose fits that picture and helps make the distinction from type 2. It comes up more often than you would think: of people diagnosed with type 2, an estimated 15 percent actually have LADA and 1 to 5 percent MODY.
Do I need to fast for a C-peptide test?
Yes. At least 8 hours without food and preferably 12, water only. Anyone who has eaten measures production as a response to that meal instead of the starting situation. Have the glucose taken in the same draw, because without that value C-peptide cannot be read.
Which tests include C-peptide?
C-peptide is part of Advanced, the broader panel. It is not orderable yet.
Related biomarkers
Read on
Sources
- 1.NHG guideline on type 2 diabetes (M01), Dutch College of General Practitioners. 2026. richtlijnen.nhg.org
- 2.Jones and Hattersley, the clinical utility of C-peptide measurement in the care of patients with diabetes, Diabetic Medicine. 2013. pubmed.ncbi.nlm.nih.gov
- 3.Unilabs, Bepalingenklapper: C-Peptide. 2026. bepalingenklapper.nl
- 4.Clinical Diagnostics, Labgids: C-peptide (tevens controle op antilichamen insuline). 2026. clinicaldiagnostics.nl
Educational information only, not medical advice. Consult a healthcare professional for clinical decisions.
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C-peptide is part of Advanced, the broader panel. It is not orderable yet.
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