Turnaround times
The quoted turnaround time is from sample receipt in the laboratory, to results authorisation in the Laboratory Information Management system. The times do not include transport of specimen to the laboratory or the administrative process to print and post/email reports. Service users must allow for transport and reporting time when ordering tests.
Clinical background:
C-peptide is formed as a by-product of the processing of insulin in the pancreatic ß cell. Proinsulin is cleaved to form equimolar amounts of mature insulin and c-peptide which are released into the circulation. So called because it connects the A and B chains of insulin in the proinsulin molecule, c-peptide is a single chain of 31 amino acids (Mol Wt. 3020D). Because c-peptide has a longer half-life than insulin (2-5 times), higher concentrations of C-peptide persist in the peripheral circulation, and these levels fluctuate less than insulin. For these reasons, c-peptide concentrations may reflect pancreatic insulin secretion more reliably than the level of insulin itself.
C-peptide measurement may be used for the following clinical applications:
Investigation of hypoglycaemia
Assessment of residual beta cell function to distinguish between type 1 and type 2 diabetes or to assess the requirement for progression to insulin therapy in type 2 diabetes.
As a marker for residual pancreatic tissue after pancreatectomy. In the case of insulinoma, C-peptide measurement may be used to detect metastasis and the response to therapy. It may also be used to monitor the progress of pancreas or islet cell transplantation.
Specimen container paediatric:
Serum or plasma (LiHep or EDTA)
Specimen container adult:
Serum or plasma (LiHep or EDTA)
Minimum volume paediatric:
1 mL blood
Minimum volume adult:
1 mL blood
Special requirements:
Samples should be separated within 6 hours of sampling.
Fasting, unless part of dynamic function test, or during spontaneous hypoglycaemic episode.
Also send glucose sample (fluoride/oxalate).
Sample stability:
Unseparated: 6 hours
Separated:3 days at 2-8°C, 2 months at -20°C
Transport requirements :
External locations: send as frozen serum/plasma.
Internal samples: send unseparated at ambient temperature to be received within 6 hours of sampling.
Interpretation:
When samples are taken during a hypoglycaemic episode, suppressed c-peptide concentrations in the presence of elevated insulin concentrations indicate exogenous insulin administration. Where both insulin and c-peptide are not appropriately suppressed, this indicates endogenous hyperinsulinism (e.g. due to the presence of an insulinoma or sulphonylurea administration). The insulin assay used in Newcastle has broad specificity and detects commonly used insulin analogues.
Measurement of c-peptide may be useful in the classification of type 1/type 2 diabetes where there is uncertainty. Utility is greatest 3-5 years after diagnosis when persistence of substantial c-peptide production suggests type 2 or monogenic diabetes. During the first 3-5 years there is considerable overlap in c-peptide concentrations between type 2 and type 1 diabetes, however absent c-peptide at any time confirms absolute insulin requirement regardless of aetiology.
As the kidney is the major site for C-peptide metabolism, patients with severe renal insufficiency may have abnormally high circulating C-peptide levels.
Reference ranges:
Adult: 0.34 – 1.80 nmol/L
Factors affecting result:
N.B. Heterophilic antibodies can interfere with immunoassays.
Other info:
EDTA and LiHep plasma also suitable