C-Peptide - Blood Tests For Diabetes

What is c-peptide?

The C peptide (Connecting peptide) is part of proinsulin and is laboratory-determined primarily in the context of the differential diagnosis of diabetes mellitus, but also for the diagnosis of an insulinoma or a factitious hypoglycaemia.


The C peptide consisting of 31 amino acids and connects the A-chain to the B-chain of the proinsulin. In the conversion of proinsulin to insulin this is enzymatically cleaved to be then distributed together with insulin from the β cells of the pancreas into the blood. C peptide has its own effects on carbohydrate metabolism and its absence may play a role in the development of diabetic organ damage. In drug-administered insulin the C-peptide is no longer available.


The C peptide can be detected in the laboratory diagnostics serum or urine by an enzymatic immunoassay. The half-life of C peptide is ten times longer than that of insulin.


The C peptide serves as a marker (measured value) of the secretion of β-cells of the pancreas. As part of a glucose load tests can their responsiveness can be determined.


Normal values ​​depending on the laboratory
- 1.1 to 5.0 ug / l
- 30 - 60 min after glucose load: 4.0 to 8.0 ug / l
Decreased values
- Diabetes mellitus type 1, for example, also LADA
- Diabetes mellitus type 2 in case of failure of insulin production in the late stage of the disease
Increased values
- Diabetes mellitus type 2 as a sign of insulin resistance
- Insulinoma
Normal values ​​in clinically proven hypoglycaemia
- Hypoglycaemia factitia as part of a Munchausen syndrome.

Relation of C peptide to fasting blood sugar
To determine the need for insulin for diabetics, the ratio of C-peptide to fasting blood glucose (FBG) can be determined. Values ​​below 11.7 talk for a secondary failure and the need for insulin administration.

Glucose tolerance test
The glucose tolerance test is used to determine the glucose stability. For this purpose, after intravenous bolus injection of glucose, the increase in C-peptide are measured (glucose). When a missing or only small fluctuations in blood glucose levels rise despite intensive therapy and good patient compliance (compliance) are probable.


The presence of C-peptide in the blood gives information on the state of the body's insulin production. The C-peptide amount is directly proportional to the undistributed insulin. The significance of diabetes mellitus in slightly decreased values ​​is low. Only the absolute lack or markedly decreased values ​​(0.5-1.0 ug under / l) correlated with a substitution requiring insulin deficiency symptoms. On the other hand can also be necessary if increased levels of insulin therapy due to insulin resistance. When a drop below the normal range, an insulin therapy, perhaps if only low doses are started, because at a still low intrinsic secretion of insulin the survival of functional β-cells is prolonged by the early exogenous insulin administration. Insofar as the height of the C-peptide level has a prognostic value on the course of diabetes mellitus disease.

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

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