What is the C-peptide?
The C-peptide is produced in the pancreas during the formation of insulin: the so-called beta cells produce the inactive precursor proinsulin. To activate it, it is split – into the blood sugar-lowering hormone insulin and the C-peptide. The term stands for Connecting Peptide, because it links the building blocks of proinsulin.
Unlike insulin, C-peptide is broken down much more slowly, making it an ideal measurement of pancreatic function and insulin production.
When is C-peptide determined?
In the laboratory, the C-peptide level is mainly determined to assess the performance of the pancreatic beta cells. If the beta cells are able to produce insulin, C-peptide can also be detected. An assessment of pancreatic performance is also important for therapy planning in diabetes – that is, in deciding whether a diabetic needs to inject insulin or not.
Very rarely, hypoglycemia can be diagnosed as hypoglycaemia factitia. This is a mental illness in which patients deliberately lower their blood glucose levels with insulin. In this way, those affected usually want to gain increased attention and care from doctors, hospitals or relatives. In this particular case, C-peptide levels are normal, while insulin is too high and blood glucose is too low. If the patient uses sulfonylureas to lower blood glucose, C-peptide and insulin are elevated.
C-peptide – normal values
As a rule, the laboratory value is measured in a fasting state. The following normal values apply:
|
Conditions |
C-Peptide: Norm |
|
12-hour fasting |
0.7 – 2.0 µg/l |
|
prolonged fasting |
< 0.7 µg/l |
|
Maximum values under glucose or glucagon stimulation |
2.7 – 5.7 µg/l |
Glucose or glucagon stimulation is performed to assess whether a diabetic patient needs to inject insulin. This is done by giving the patient glucose or glucagon before measuring the C-peptide level.
When is the C-peptide low?
The C-peptide is naturally low when the pancreas does not have to produce insulin, i.e. the blood glucose level is low and the patient has not eaten anything.
Other possible causes of a decreased C-peptide are Addison’s disease and the administration of certain drugs (alpha-sympathomimetics).
When is the C-peptide elevated?
When carbohydrate-rich or sugar-rich food is consumed, the pancreas secretes insulin and at the same time C-peptide to stimulate the body’s cells to absorb blood sugar. The laboratory value is then naturally elevated.
In the early stages of type 2 diabetes mellitus, C-peptide is also elevated. This is due to the fact that in affected individuals, the body’s cells are increasingly resistant to insulin, i.e. they respond little or not at all to its signal to absorb blood glucose. In response, the beta cells produce more and more insulin and C-peptide until they are finally exhausted and production ceases.
Much less frequently, insulinomas are the cause of an increase in C-peptide. Other possible causes are kidney weakness (renal insufficiency), metabolic syndrome, and treatment with corticosteroids.
What to do if C-peptide is elevated or decreased?
Treatment depends on the cause of the altered laboratory values. Your doctor will discuss the measurement results and also the further therapy with you.
If an insulinoma is causing elevated C-peptide levels, it is surgically removed if possible.
