Diagnostic work-up of hypoglycemia:

-  Diagnosis of factitious hypoglycemia due to surreptitious

   administration of insulin

-  Evaluation of possible insulinoma

-  Surrogate measure for the absence or presence of physiological

   suppressibility of endogenous insulin secretion during diagnostic

   insulin-induced hypoglycemia (C-peptide suppression test)

Assessing insulin secretory reserve in selected diabetic patients

(as listed below) who either have insulin autoantibodies or who are

receiving insulin therapy

-  Assessing residual endogenous insulin secretory reserve

-  Monitoring pancreatic and islet cell transplant function

-  Monitoring immunomodulatory therapy aimed at slowing

   progression of preclinical, or very early stage type 1 DM

C-peptide (connecting peptide), a 31-amino-acid polypeptide,

represents the midportion of the proinsulin molecule. Proinsulin

resembles a hairpin structure, with the N-terminal and C-terminal,

which correspond to the A and B chains of the mature insulin molecule,

oriented parallel to each other and linked by disulfide bonds. The

looped portion of the hairpin between the A and B chains is called

C-peptide. During insulin secretion it is enzymatically cleaved off and

cosecreted in equimolar proportion with mature insulin molecules.

Following secretion, insulin and C-peptide enter the portal circulation

and are routed through the liver, where at least 50% of the insulin

binds to receptors, initiates specific hepatic actions (stimulation of

hepatic glucose uptake and suppression of glycogenolysis,

gluconeogenesis, and ketogenesis) and is subsequently degraded.

Most of the insulin molecules that pass through the liver into

the main circulation bind to peripheral insulin receptors, promoting

glucose uptake, while the remaining molecules undergo renal

elimination. Unlike insulin, C-peptide is subject to neither hepatic

nor significant peripheral degradation, but is mainly removed by

the kidneys. As a result, C-peptide has a longer half-life than

insulin (30-35 minutes versus 5-10 minutes) and the molar ratio

of circulating insulin to circulating C-peptide is generally <1,

despite equimolar secretion. Until recently, C-peptide was

thought to have no physiological function, but it now appears that

there may be specific C-peptide cell-surface receptors (most likely

belonging to the super-family of G-protein coupled receptors),

which influence endothelial responsiveness and skeletal and

renal blood flow.

In most disease conditions associated with abnormal

serum insulin levels, the changes in serum C-peptide levels

parallel insulin-related alterations (insulin to C-peptide molar

ratio < or =1). Both serum C-peptide and serum insulin levels are

elevated in renal failure and in disease states that lead to

augmented primary endogenous insulin secretion (eg, insulinoma,

sulfonylurea intoxication). Both also may be raised in any

disease states that cause secondary increases in endogenous

insulin secretion mediated through insulin resistance, primarily

obesity, glucose intolerance, and early type 2 diabetes mellitus

(DM), as well as endocrine disorders associated with

hypersecretion of insulin-antagonistic hormones (eg, Cushing's

syndrome, acromegaly). Failing insulin secretion in type 1 DM

and longstanding type 2 DM is associated with corresponding

reductions in serum C-peptide levels.

Discordant serum insulin and serum C-peptide abnormalities are

mainly observed in 2 situations: exogenous insulin administration

and in the presence of anti-insulin autoantibodies. Factitious

hypoglycemia due to surreptitious insulin administration results in

appropriate suppression of endogenous insulin and C-peptide

secretion. At the same time, the peripherally administered insulin

bypasses the hepatic first-pass metabolism. In these situations,

insulin levels are elevated and C-peptide levels are decreased.  

In patients with insulin antibodies, insulin levels are increased

because of the prolonged half-life of autoantibody-bound insulin.

Some patients with anti-idiotypic anti-insulin autoantibodies

experience episodic hypoglycemia caused by displacement of

autoantibody-bound insulin.

0.9-4.3 ng/mL

297-1,419 pmol/L

To compare insulin and C-peptide concentrations (ie, insulin

to C-peptide ratio), convert insulin to pmol/L: insulin concentration

(in uIU/mL) x 7.18 = insulin concentration (in pmol/L).

Factitious hypoglycemia due to surreptitious insulin administration

results in elevated serum insulin levels and low or undetectable

C-peptide levels, with a clear reversal of the physiological molar

insulin to C-peptide ratio (< or =1) to an insulin to C-peptide ratio of >1.

By contrast, insulin and C-peptide levels are both elevated in

insulinoma and the insulin to C-peptide molar ratio is < or =1.

Sulfonylurea ingestion also is associated with preservation of the

insulin to C-peptide molar ratio of < or =1.

In patients with insulin autoantibodies, the insulin to C-peptide ratio

may be reversed to >1, because of the prolonged half-life of

autoantibody-bound insulin.

Dynamic testing may be necessary in the work-up of hypoglycemia;

the C-peptide suppression test is most commonly employed.

C-peptide levels are measured following induction of hypoglycemia

through exogenous insulin administration. The test relies on the

demonstration of the nonsuppressibility of serum C-peptide levels

within 2 hours following insulin-induced hypoglycemia in patients

with insulinoma.

There are currently no established pediatric reference ranges for

serum C-peptide levels.

Significant hemolysis will result in artifactually lower C-peptide levels

and such specimens are usually rejected. However, even mild

hemolysis can lead to modest decrements in C-peptide values.

There is significant (>20%) cross-reactivity between C-peptide and

proinsulin. There is no significant cross-reactivity with other pancreatic

islet cell peptides or neuroendocrine peptides.

Very high C-peptide levels (>180 ng/mL) may result in artifactually

low measurements (hook effect). Such levels are very unlikely to

occur in patients, but if individuals are suspected of having serum

levels >180 ng/mL, the laboratory should be alerted in order to

allow dilution of the specimen prior to testing.

This assay uses 2 mouse-derived monoclonal antibodies and

may, therefore, be prone to interference by heterophile

antimouse antibodies (HAMA). The lab should be alerted to

suspected or known HAMA-positive specimens in order to allow

the use of heterophile antibody blocking tubes for such specimens.

In the assessment of hypoglycemia, neither C-peptide nor insulin

measurements are useful or indicated if serum blood sugar levels

exceed 60 mg/dL.

In the diagnosis and management of DM, measurement of serum

insulin levels usually provides superior information to that of

serum C-peptide.

Patients with a body mass index (BMI) >25 may have elevated

fasting C-peptide levels.

1.   Service FJ, O'Brien PC, Kao PC, Young WF Jr:  C-peptide

      suppression test:  effects of gender, age, and body mass

      index; implications for the diagnosis of insulinoma.  J Clin

      Endocrinol Metab 1992;74:204-210

2.   Lebowitz MR, Blumenthal SA:  The molar ratio of insulin to

      C-peptide. An aid to the diagnosis of hypoglycemia due to

      surreptitious (or inadvertent) insulin administration. Arch

      Int Med 1993 Mar 8;153(5):650-655

3.   Service FJ:  Hypoglycemic disorders. N Engl J Med 1995

      Apr 27;332(17):1144-1152

4.   Wahren J, Ekberg K, Johansson J, et al:  Role of C-peptide

      in human physiology. Am J Physiol Endocrinol Metab 2000

      May;278(5):E759-E768