C-peptide

How you prepare depends on why the test is being done. You cannot do a C‑peptide test at home and it is not performed in every laboratory, so your sample may be sent to a specialised laboratory to measure C‑peptide. Your doctor or nurse will give you specific instructions. A blood sample or a urine sample may be taken. There are three common scenarios: 

Fasting test: You may be asked not to eat or drink anything other than water for 8 to 10 hours before the test. This is the most common preparation for a C‑peptide blood test. 

Meal-stimulated test: For some urine tests, you will be asked to eat your largest meal of the day and provide a urine sample two hours later. Urine testing may be used under certain circumstances, mainly to be used in patients on insulin treatment, but blood testing is still the main sample collection method to be used in everyone. This stimulates the pancreas to release insulin and C‑peptide. 

During a hypoglycaemic episode: If the test is being used to investigate low blood sugar, it may be taken during or shortly after a hypoglycaemic episode, when blood glucose is low. Your doctor will advise you on what to do. 

Always follow the specific instructions you are given. Taking the test under the wrong conditions can affect the result. 

What the test measures 

The test measures the amount of C‑peptide in a blood or urine sample. Initially, a large molecule called proinsulin is made by special cells in the pancreas called beta cells, which is then broken down to create insulin. When your pancreas creates insulin, it also produces an equal amount of C‑peptide as a by-product. The two are released into the bloodstream together. Insulin is broken down quickly by the liver. So, C‑peptide stays in the blood longer than insulin and is often a more reliable measure of how much insulin your body has produced. Importantly, C‑peptide only comes from your own pancreas. It is not present in injected insulin, so measuring C‑peptide allows doctors to tell the difference between insulin your body makes (endogenous insulin) and insulin you have injected (exogenous insulin).

C‑peptide measures insulin made by your own pancreas only. If you inject insulin, the C‑peptide test is not affected by those injections. This makes it a useful way to check how much insulin your body is still producing, even if you are on insulin therapy.

What your results mean 

If your C‑peptide is low or undetectable 

A low or undetectable C‑peptide level means your pancreas is producing little or no insulin. This is typical in: 

• Type 1 diabetes — the immune system has destroyed the insulin-producing beta cells, so the pancreas can no longer make insulin. People with type 1 diabetes depend entirely on injected insulin. 
• Late-stage type 1 or type 2 diabetes — after many years, the beta cells may be significantly depleted, reducing or stopping insulin production. 
• After surgical removal of the pancreas (pancreatectomy). 
• Patients on injected insulin, as the injected insulin will suppress production of insulin and C‑peptide. 

A low C‑peptide during a hypoglycaemic episode — alongside low blood glucose — suggests the low blood sugar was not caused by excessive insulin production from the pancreas. It may point to other causes such as alcohol, medications or exogenous insulin, some endocrine conditions, liver disease, insulinomas. 

If your C‑peptide is normal or high 

A normal or raised C‑peptide level means your pancreas is still producing insulin. High C‑peptide levels suggest the body is making more insulin than expected. This can be seen in: 

• A normal response to high concentrations of glucose intake. 
• Type 2 diabetes or insulin resistance — the body produces extra insulin to try to overcome resistance in the cells. High C‑peptide can indicate that insulin resistance is present. 
• Early type 2 diabetes — where the pancreas is working harder to compensate for reduced effectiveness of insulin 
• Insulinoma — a tumour in the pancreas that produces excess insulin continuously. A high C‑peptide alongside low blood glucose during a hypoglycaemic episode is a key clue to this diagnosis. 
• Other conditions — including low blood potassium, Cushing’s syndrome, kidney disease, and pregnancy, or during an Oral Glucose Tolerance Test (OGTT) which can all raise C‑peptide levels 

C‑peptide and diabetes type 

C‑peptide is particularly useful when the type of diabetes is unclear. In general: 

• A very low or undetectable C‑peptide supports a diagnosis of type 1 diabetes — the pancreas has stopped producing insulin 
• A normal or high C‑peptide suggests type 2 diabetes or another form (when other features of diabetes are present) — the pancreas is still producing insulin, but it may not be working effectively 

In practice, diabetes typing is complex and C‑peptide is used alongside other tests — including diabetes antibody tests — to reach the right diagnosis. Your doctor will explain what your result means in your specific situation. 

The diagnosis / classification of MODY (Maturity Onset Diabetes of the Young) and LADA (latent autoimmune diabetes in adults) are more complex and is often supported by measuring C‑peptide. 

C‑peptide and hypoglycaemia investigation 

If you have episodes of low blood sugar, your doctor may compare your C‑peptide level with your blood glucose level at the same time. Symptoms of low blood glucose include sweating, palpitations, hunger, confusion, visual problems and seizures, however, these can also occur in other conditions. The combination of results helps identify the cause: 

• A high C‑peptide with low blood glucose points to excess insulin production from the pancreas — possibly due to diabetes medications that stimulates pancreatic insulin production, or more rarely, an insulinoma. 
• A low C‑peptide with low blood glucose suggests the low blood sugar is not due to the pancreas overproducing insulin. The cause may be injected insulin, alcohol, liver disease, or another reason. 

Reference ranges 

Reference ranges for C‑peptide vary between laboratories and depend on the testing method and the conditions under which the sample was taken (fasting, after a meal, or during hypoglycaemia). There is typically 5 times as much C‑peptide in the bloodstream than insulin, since C‑peptide stays in the blood for longer periods of time than insulin, so it is important to use the reference ranges for each test to understand the results. Your lab report will include the reference range used for your sample. 

Because C‑peptide is cleared by the kidneys, results can be higher than expected in people with kidney disease. Your doctor will take this into account when interpreting your result. 

What happens after your C‑peptide test depends on why it was done and what the result shows. 

If the result suggests your pancreas is producing little or no insulin, your doctor may confirm a diagnosis of type 1 diabetes or discuss starting insulin treatment. If the result suggests high insulin production or insulin resistance, your doctor may review your diabetes medicines, discuss lifestyle changes, or arrange further tests. 

If the test was done to investigate low blood sugars, the result will help guide further investigation to find out the cause. Your doctor will work with you to explain the next steps and what the results mean for your care. 

Several things can affect C‑peptide levels and make results harder to interpret: 

• Kidney disease — C‑peptide is cleared by the kidneys. Reduced kidney function means C‑peptide builds up in the blood, giving a falsely high result 
• Liver disease — can affect insulin clearance, which influences how C‑peptide and insulin compare 
• Fasting or eating — C‑peptide levels change with food intake. Results must be interpreted in the context of whether you were fasting or had eaten 
• Alcohol — can suppress insulin production and lower C‑peptide 
• High-dose biotin (vitamin B7) supplements — in some assays, very high biotin levels can interfere with C‑peptide measurement 
• Diuretics (water tablets) — can affect C‑peptide levels in some cases 
• Pregnancy — can raise C‑peptide levels 

The test is not performed in every laboratory and often needs to be sent to a specialist laboratory. This may mean results take longer than a standard blood test.

C‑peptide is usually requested alongside other tests. Depending on your situation, your doctor may also request: 

• Glucose test – blood glucose is almost always measured at the same time as C‑peptide, especially when investigating hypoglycaemia or diabetes 
• Insulin test – measuring insulin directly can provide additional information, particularly in hypoglycaemia investigations 
• HbA1c – a measure of average blood glucose over the past two to three months; used to monitor diabetes control 
• Diabetes antibody tests (islet autoantibodies) – such as GAD antibodies, IA‑2, and ZnT8 antibodies; used to help confirm type 1 diabetes 
• Glucose tolerance test (OGTT) – may be done to assess insulin resistance or to stimulate the pancreas and measure C‑peptide response 
• Kidney function tests or liver function tests– to check whether kidney or liver disease may be affecting C‑peptide or insulin clearance. 
• Imaging tests (ultrasound, CT, or MRI) – if an insulinoma is suspected, imaging of the pancreas is needed to look for a tumour 

Diabetes is a condition where blood glucose levels are too high because the body does not produce enough insulin or does not use it effectively. There are several types, including type 1 and type 2 diabetes. Treatment depends on the type and may include lifestyle changes, tablets, and insulin injections.