C-peptide: what it is, what it tells you and when it’s tested

When your pancreas makes insulin, it releases a small molecule at the same moment: C-peptide. One comes out for every insulin. Measuring it shows how much insulin your body is making, even if you inject insulin, because injected insulin contains no C-peptide.

When it’s tested

Doctors mostly order it in people who already have diabetes, to work out which type: in type 1 the pancreas makes little or no insulin, in type 2 it often still makes plenty. It is most useful a few years after diagnosis, and it helps choose the treatment.

It is also used to understand unexplained low blood sugar: then it is measured during the episode, together with glucose and insulin.

It is not a routine test for people who feel well. That is why Rosa does not test it. Instead it measures glucose, insulin and HbA1c, which show how you handle sugar and whether insulin is struggling.

What it tells you, and what it does not

It shows how much insulin you make. It is steadier than insulin in the blood and is not affected by injections.

It is not read on its own. A high value with normal glucose means your pancreas is working hard to keep it down. A low value with high glucose means your pancreas cannot keep up.

Your kidneys clear about half of it. If they filter less, C-peptide rises even without more insulin.

That is why it is read together with other tests:

• Glucose: Always read with the sugar from the same sample
• Insulin: Together they show how much insulin you make and use
• HbA1c: It shows your average sugar over recent months
• eGFR: If your kidneys filter less, C-peptide rises
• Glucose tolerance test: It is sometimes measured during the test

Normal C-peptide levels

The lab decides what is normal. Many lab reports give 0.8 to 4.1 ng/mL when fasting. Others use slightly different limits, or another unit such as pmol/L or nmol/L. The range on your own report is what counts.

After a meal C-peptide rises, because insulin rises. That is why it matters whether you had fasted.

In people with diabetes who use insulin, doctors use their own thresholds, different for fasting and after meals.

What your result means

Find the row for your fasting result, in ng/mL. Always read it with the glucose from the same sample: a value outside the limits should be seen by a doctor.

• Below 0.8: Low. Your pancreas is making little insulin. It happens in type 1 diabetes and in some long-standing diabetes. It is read with the glucose taken at the same time. Talk to your doctor, especially if your glucose is high.
• 0.8 to 4.1: Normal. Your pancreas is making a usual amount of insulin, when fasting. Nothing to do, if your glucose is normal too.
• Above 4.1: High. Your pancreas is making a lot of insulin, often because your body responds poorly to it. It also rises if your kidneys filter less, or if you had not fasted. Talk to your doctor: they usually look at glucose, HbA1c and your kidneys.

If your C-peptide is high or low

• Low C-peptide: Your pancreas is making little insulin. With high glucose it points to type 1 or long-standing diabetes.
• High C-peptide: Your pancreas is making a lot of insulin, often because of insulin resistance. Kidneys or a non-fasting sample also raise it.

The most common causes

The causes depend on the direction.

• Low: type 1 diabetes: The immune system destroys the cells that make insulin.
• Low: long-standing diabetes: Over the years the pancreas can make less and less.
• Low with low glucose: If glucose is low, insulin is high and C-peptide is low, the insulin is coming from outside, from injections.
• High: insulin resistance: Excess weight, little exercise, early type 2 diabetes. The pancreas compensates by making more.
• High: kidneys filtering less: C-peptide builds up even without more insulin.
• High: rare causes: Some diabetes medicines that stimulate the pancreas and, rarely, a benign pancreatic tumour that makes insulin.

What can skew the result

A few things move it without your pancreas really changing.

• Not fasting: It rises after a meal. If your doctor wants it fasting, arrive having fasted for at least eight hours.
• Your kidneys: If they filter less, the value rises. Your doctor reads it with creatinine and eGFR.
• Some diabetes medicines: Those that stimulate the pancreas raise it. Tell your doctor, and do not stop them on your own.

When to worry

Talk to your doctor if:

• Your C-peptide is low and your glucose is high.
• You are always thirsty, pass a lot of urine and lose weight without trying.
• You have episodes of cold sweat, shaking, sudden hunger or confusion that pass when you eat.
• It is high and your glucose or HbA1c is borderline too.
• You have extreme thirst with vomiting, heavy breathing or drowsiness, or very low blood sugar that will not come up: call 112 or go to the emergency department.

Frequently asked questions

What are normal C-peptide levels? Many lab reports give 0.8 to 4.1 ng/mL when fasting. Others use different limits or units. The range on your own report is what counts.

Low C-peptide: what does it mean? That your pancreas is making little insulin. With high glucose it points to type 1 or long-standing diabetes. Your doctor reads it with the other tests.

High C-peptide: what does it mean? Usually that your pancreas is making a lot of insulin because your body responds poorly to it. It also rises if your kidneys filter less or if you had not fasted.

What is the difference between C-peptide and insulin? They leave the pancreas together, one for one. Insulin, though, is cleared faster and is affected by injections. C-peptide only measures the insulin you make yourself.

Does Rosa test C-peptide? No. Rosa measures glucose, insulin and HbA1c, which show how you handle sugar. If your doctor wants C-peptide, they order it separately.

Sources

• Ospedali Riuniti University Hospital, Trieste, laboratory medicine department handbook: reference intervals. AOU Trieste, manuale del laboratorio
• Gemelli University Hospital, Rome, clinical chemistry reference values, 2024. Policlinico Gemelli, valori di riferimento 2024
• Jones and Hattersley, the clinical utility of C-peptide measurement in the care of patients with diabetes. Diabetic Medicine, 2013. PMID 23413806
• Cryer and colleagues, evaluation and management of adult hypoglycemic disorders: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology and Metabolism, 2009. PMID 19088155