Diagnostic Use
C-peptide is the 31 amino acid peptide segment released from proteolytic breakdown of Proinsulin to Insulin in pancreatic beta cells. There is equimolar secretion of C-peptide and insulin into the portal vein. However, in the systemic circulation the fasting C-peptide level is about 5 – 15 times higher than insulin, due to negligible extraction in the liver and peripheral tissues. C-peptide is primarily degraded in the kidneys so its level is elevated in renal failure.
In hypoglycaemic states, C-peptide level can help to identify the cause: patients with endogenous overproduction of insulin (e.g. insulinoma or sulphonylurea intoxication) have high insulin as well as high C-peptide levels (Insulin to C-peptide molar ratio ≤1) whereas those with exogenous surreptitious insulin administration or anti-insulin autoantibodies have high insulin but low C-peptide level, i.e. Insulin to C-peptide molar ratio >1. To convert Insulin in mU/L to pmol/L, multiply insulin result by 6.95.
Interpretation
C-peptide testing should not be used to diagnose diabetes mellitus. In both type 1 diabetes or long standing type 2 diabetes with absolute insulin deficiency, the C-peptide level can be low. In a patient with recently diagnosed diabetes or insulin-treated diabetes in whom the diagnosis of T1D is being considered, a random C-peptide <50 pmol/L, in the presence of glucose >7 mmol/L, reduces the need for pursuing antibody support for a clinical diagnosis of T1D. Please note that the testing of antibodies for prediction of T1D (in those without diabetes) is not clinically indicated, and should only be done within clinical trial settings.
C-peptide measurement can also be useful in follow up evaluations after pancreatectomy and post-pancreatic transplantation to measure pancreatic beta cell secretory reserve. C-peptide measurement after an overnight 8 hour fast or mixed meal tolerance test can help to assess beta cell secretory capacity.
In patients with insulin resistance, early type 2 diabetes mellitus or endocrine disorders like Cushing’s syndrome and Acromegaly, both insulin and C-peptide levels are raised. However, testing C-peptide level under these settings is generally not clinically useful and will need prior approval.
Reference Intervals
Units : pmol/L
Reference interval (fasting): 370 – 1470 (after overnight fast)
ng/mL x 333 = pmol/L
Test Method
Test performed by: LabPlus Automation
Principle: Chemiluminesent (ECLIA)
Reagents: Roche Assay
Analyser: Roche Cobas