C-Peptide (Serum)

Synonym

Tubes

Additional information

The patient is instructed to fast for 8-10 hours (except water) before test
Sample drawn in a chilled tube. Allow specimen to clot completely, separate serum/plasma in a separate plastic vial ASAP, pack in ice, and sent to lab immediately
Handle samples gently to prevent hemolysis
Sample for blood glucose is usually drawn simultaneously

Info

The C-Peptide test measures the concentration of C-peptide, a peptide, which is produced along with the production of the hormone insulin and found in the blood
Proinsulin, an inactive precursor of insulin, is synthesized in the beta cells of the pancreas, which gets cleaved into insulin and biologically inactive C-peptide. As insulin is released into the bloodstream, C-peptide chain splits off from the hormone (C-peptide connects the A and B chains of insulin)
C-peptide levels usually correlate with insulin levels and provide a reliable indication of endogenous release of insulin, as well as how well pancreatic beta cells are secreting insulin

Clinical

Evaluation of hypoglycemia
To determine insulin secretion and beta-cell function in patients with diabetes mellitus (type 1)
C-peptide assay provides distinction between exogenous and endogenous circulating insulin
Detection of factitious cause of hypoglycemia such as excessive insulin administration
Evaluation and measurement of insulin secretion in presence of circulating insulin antibodies
Differential diagnosis of insulinoma versus factitious hypoglycemia
Aids in the diagnosis of insulinoma and to monitor the patient's recovery after excision of an insulinoma, tumor recurrence, or metastases
To detect residual tissue after total pancreatectomy for carcinoma

Patients with insulin-dependent diabetes mellitus (type I) Usually have no or very low concentrations of C-peptide (presence of anti-insulin antibodies), whereas patients with non-insulin dependent diabetes mellitus (type II) tend to have normal or elevated C-peptide levels
In insulinoma, C-peptide and insulin levels are increased with insulin/C-peptide ratio of <1.0, indicating endogenous insulin secretion
In factitious hypoglycemia due to insulin injection, insulin levels are increased and C-peptide levels are decreased, with insulin/C-peptide ratio of >1.0, which reflects exogenous insulin. However, drugs that promote insulin secretion will result in an elevated C-peptide and insulin

Additional information:

Though equimolar amounts of insulin and C-peptide are secreted, insulin is processed and eliminated mostly by the liver, while the kidneys excrete C-peptide. The half life of C-peptide is about 30 minutes, and for insulin, about 5 minutes. As a result, C-peptide is 5 times more than the insulin in the bloodstream in normal conditions
Normal day-to-day variation is about 10-25%, with increasing levels with age >60 yrs
Levels are higher in Hispanics than in Whites
C-peptide-stimulating test aids in distinguish between insulin-dependent (type 1) and non-insulin-dependent (type 2) diabetes
C-peptide and endogenous insulin levels do not always correlate in obese patients
Factors interfering with test results include:

Radioisotope testing within a week before test
Hemolysis due to rough handling of sample
Failure to follow pretest restrictions such as dietary restrictions before the procedure
Failure to pack sample in ice and promptly send to lab
Radioimmunoassay (RIA) cross-reacts with proinsulin, thereby different antibodies show considerable difference in cross-reactivity, which makes interlaboratory comparison difficult

Nl Result

Consult your laboratory for their normal ranges as these may vary somewhat from the ones listed below.

High Result

Conditions associated with elevated levels of C-peptide levels include:

Low Result

Conditions associated with decreased C-peptide levels include:

References

ARUP Laboratories®. C-Peptide, Serum or Plasma. [Homepage on the internet]©2007. Last accessed on July 24, 2007. Available at URL: http://www.aruplab.com/guides/ug/tests/0070103.jsp
Denner L et al. Directed engineering of umbilical cord blood stem cells to produce C-peptide and insulin. Cell Prolif. 2007 Jun;40(3):367-80.
eMedicine from WebMD®. Hypoglycemia. [Homepage on the Internet] ©1996-2007. Last updated on October 10, 2006. Last accessed on July 24, 2007. Available at URL: http://www.emedicine.com/emerg/topic272.htm
Jensen RA et al. Multiple factors affect the loss of measurable C-peptide over 6 years in newly diagnosed 15- to 35-year-old diabetic subjects. J Diabetes Complications. 2007 Jul-Aug;21(4):205-13.
Lab Tests online®. C-peptide. [Homepage on the Internet]©2001-2007. Last reviewed on December 16,. Last accessed on July 24, 2007. Available at URL: http://www.labtestsonline.org/understanding/analytes/c_peptide/glance.html
Laboratory Corporation of America®. C-Peptide, Serum. [Homepage on the internet]©2007. Last accessed on July 24, 2007. Available at URL: http://www.labcorp.com/datasets/labcorp/html/chapter/mono/sr001400.htm
Vezzosi D et al. Insulin, C-peptide and proinsulin for the biochemical diagnosis of hypoglycaemia related to endogenous hyperinsulinism. Eur J Endocrinol. 2007 Jul;157(1):75-83.