Test Background Pro-insulin is cleaved to form connecting peptide (C-peptide) and insulin prior to secretion into the portal circulation. They are secreted in equi-molar concentrations, however C-peptide is metabolised more slowly than insulin. C-peptide can be helpful in the differential diagnosis of type 1 and 2 diabetes mellitus. The insulin:C-peptide ratio can be used to diagnose use of exogenous insulin.
Evaluation of fasting hypoglycaemia: β-cell tumours or factitious causes, eg. abuse of sulphonylureas, exogenous insulin, Classification of diabetes mellitus: in type 1, C-peptide is decreased while, in type 2, C-peptide is normal/raised, Diagnosis of insulinoma, Monitoring purposes (following pancreatectomy or pancreal/islet cell transplant).
Reference Range C-peptide (pmol/L) ratio to insulin (mIU/L): 30-70
Sample Required Serum (red top) on ice rush to laboratory
Sample Collection Collect into tube on ice, separate in a chilled centrifuge and freeze immediately.
Sample Volume 1 mL
Transport/Storage Send to lab frozen.
Turnaround Time 1 week
Notes Grossly haemolysed samples are unsuitable for this assay.