C-peptide (connecting peptide) is a peptide (protein) of 31 amino acids that connects the two chains of proinsulin (A-/B-chain), the precursor of insulin. Proinsulin is cleaved into insulin and C-peptide. Thus, it plays an important role in the regulation of glucose metabolism (blood sugar).
C-peptide concentration is considered an indicator of beta cell function. Beta cells are the insulin-producing cells in the islets of Langerhans of the pancreas (pancreas).
The procedure
Material needed
- Blood serum
Preparation of the patient
- Collection after 12-hour fasting
- Collection after glucose/glucagon stimulation – to determine residual insulin synthesis.
Confounding factors
- Unknown
Standard values
| Blood collection | Normal value in μg/l |
| After 12-hour fasting | 0,7-2,0 |
| After prolonged fasting | < 0,7 |
Conversion factor
- 1 μg/l = ng/ml
Normal value – after stimulation with glucagon.
| C-peptide basal in μg/l | C-peptide 6 min after glucagon administration μg/l | Evaluation |
| < 0,7 | < 1,0 | Insulin-dependent diabetes mellitus |
| 0,7-1,8 | No increase | Statement impossible |
| > 1,8 | > 2,9 | Non-insulin-dependent diabetes mellitus |
Indications
- Suspected insulinoma – rare, usually benign (benign) tumor composed of endocrine cells (islets of Langerhans) of the pancreas (pancreas) in which increased insulin is produced
- Advanced diagnostics in diabetes mellitus (diabetes).
Interpretation
Interpretation – lowered value
- Diabetes mellitus (diabetes)
- Hunger
- Addison’s disease (primary adrenocortical insufficiency).
- After the application of alpha-sympathomimetics (nor-/epinephrine).
Interpretation – increased value
- Temporary increase after food intake.
- Insulinoma
- Metabolic syndrome – clinical name for the symptom combination of obesity (overweight), arterial hypertension (high blood pressure), insulin resistance and hyperlipidemia (lipid metabolism disorder).
- Renal function impairment
- Therapy with corticosteroids
Notes
- To assess the values, a simultaneous blood glucose (glucose) measurement must be made.
- C-peptide determination is superior to insulin determination because it is not affected by insulin antibodies or exogenous insulin administration; in addition, C-peptide has a longer half-life
- In hypoglycaemia factitia (clinical picture in which there is an intentional lowering of blood glucose (hypoglycemia) by targeted self-administration of blood glucose-lowering agents (mainly sulfonylureas)), C-peptide/insulin ratios are found well below 1.
