C peptide

The C-peptide serves as a measurement for the secretion of the beta cells of the pancreas. It is part of the primary peptide proinsulin and consists of 31 amino acids. The C-peptide connects the A-chain with the B-chain of proinsulin. When proinsulin is converted to insulin, the C-peptide is split off enzymatically. It is then secreted into the blood together with the insulin from the beta cells of the pancreas. The half-life of C-peptide is about 10 times longer than that of insulin.

Apparently, the C-peptide also has its own effects in cell metabolism. For example, it binds to G-protein-coupled receptors on the cell membrane of different cells (e.g. neurones or endothelial cells) and thus activates intracellular signalling pathways such as MAPK or PKC. Pathogenetically, its absence may possibly play a role in the development of diabetic organ damage. The C-peptide is mainly used for the laboratory diagnosis of diabetes mellitus since its value in the blood provides information on the body's own insulin production. The concentration of the C-peptide correlates directly with the insulin level.

Insulin deficiency diabetes: Complete deficiency of C-peptide or markedly decreased levels (<0.5-1.0 µg/l) correlate with insulin deficiency diabetes. In insulin resistance, C-peptide levels may be elevated.

In therapeutic insulins, the C-peptide is no longer present.

In insulinoma, C-peptide plays an important role in the diagnosis of the disease and may later serve as a tumor marker.

Hypoglycaemia factitia: The determination of the C-peptide can be useful for the detection of hypoglycaemia factitia.

Determination in serum or urine is performed by an enzymatic immunoassay. Serum is required for the determination of C-peptide. The serum should be frozen immediately if possible. If processed within 8 hours the value is relatively stable.

• Normal values: 1.1 - 5.0 µg/l
• Decreased values (basal values < 0.5 mg/dl):
• diabetes mellitus type 1, e.g. also in LADA
• Diabetes mellitus type 2 in the late stage of the disease with failure of insulin production.
• Elevated values:
• Diabetes mellitus type 2 as a sign of insulin resistance.
• Insulinoma

C-peptide in glucose stress test: The glucose stress test is used to determine glucose stability. The increase in C-peptide is measured after intravenous bolus injection of glucose (glucose). If there is no or only a small increase, fluctuations in blood glucose levels are likely despite intensive therapy and good patient cooperation (compliance). Values after glucose loading: C-peptide values 30 - 60 min after glucose loading: 4.0 - 8.0 µg/l