Synonyms

   Connecting Peptide of Insulin

Clinical Significance

   It is useful for Diagnostic workup 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 diabetes mellitus.

Specimen

   Serum

Stability

   Frozen (preferred) : 30 days

   Refrigerated : 7 days

Reference Range

   1.1 – 4.4 ng/ml

Interpretation

   A high level of C-peptide generally indicates a high level of endogenous insulin production. This may be in response to a high blood glucose caused by glucose intake and/or insulin resistance. A high level of C-peptide is also seen with insulinomas and may be seen with low blood potassium, Cushing syndrome, and renal failure. A low level of C-peptide is associated with a low level of insulin production. This can occur when insufficient insulin is being produced by the beta cells, with diabetes for example, or when production is suppressed by treatment with exogenous insulin.

   Absent C-peptide means there is an absolute requirement for insulin from an outside source. When used for monitoring someone diagnosed with an insulinoma, decreasing levels of C-peptide indicate a response to treatment. Levels that increase may indicate that a tumor has come back (recurrence). When used for managing your diabetes, the meaning of a value must be considered in context of your type of diabetes and the type of treatment you are currently receiving. With all the information, your health care practitioner can determine whether or not a change in treatment is necessary.

Limitation

   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. In the assessment of hypoglycemia, neither C-peptide nor insulin measurements are useful, or indicated, if serum blood glucose levels exceed 60 mg/dL. In the diagnosis and management of diabetes mellitus, measurement of serum insulin levels usually provides superior information to that of serum C-peptide.