EBM Consult

Lab Test: C Peptide, Insulin C Peptide (Blood) Level

    Lab Test
    • C Peptide (Blood)
    • Insulin C-peptide measurement
    • Measurement of C-peptide in blood for the evaluation and treatment of disorders related to abnormal insulin levels, such as diabetes, hypoglycemia, and insulinoma.
    • Used to identify patients who secretly self-administer insulin.
    Reference Range
    • Adults:  0.5-2.0 ng/mL (0.17-0.66 nmol/L)* (PDr)
    • Fasting:  0.78-1.89 ng/mL or 0.26-0.62 nmol/L (SI units)
    • 1 hour after glucose load:  5-12 ng/mL
    Indications & Uses
    • Determination of etiology of acute or recurrent hypoglycemia - decreased C-peptide levels of 2 ng/mL or less associated with high insulin levels in patients experiencing hypoglycemia suggest exogenous (factitious) hyperinsulinism.          Decreased C-peptide with low insulin and low glucose levels is associated with non-insulin-mediated hypoglycemias from liver disease, ethanol hypoglycemia, or adrenal insufficiency. Increased C-peptide levels > 2 ng/mL, with elevated insulin and proinsulin levels, are usually seen in patients with insulinomas or hypoglycemia secondary to oral hypoglycemic (e.g., sulfonylureas) use.
    • Patients with reactive (alimentary) hypoglycemia often present with high insulin levels; the insulin to C-peptide molar ratio will be disproportionately high.
    • Distinguishes type 1 from type 2 diabetes mellitus - Decreased C-peptide and insulin levels associated with islet cell antibodies suggest type 1 diabetes.
    • Elevated C-[e[tide and insulin levels without autoimmune markers suggest type 2 diabetes.
    • Suspected insulinoma - Increased C-[e[tide, insulin, and proinsulin levels with low beta hydroxybutyrate serum concentrations is suggestive of insulinoma. C-peptide levels of 2 nmol/L or greater usually indicate insulinoma.  The diagnosis of insulinoma is usually made during a prolonged fast using the clinical findings of Whipple's triad:  symptomatic patients with serum glucose levels less than 45 mg/dL, symptoms of hypoglycemia while fasting, and relief of symptoms following administration of glucose.
    Clinical Application

    C-peptide is a protein that connects the beta and alpha chains of proinsulin.  In the beta cells of the islet of Langerhans of the pancreas, the chains of pros=insulin are separated during the conversion of proinsulin to insulin and C-peptide.  C-peptide is released into the portal vein in nearly equal amounts.  The capacity of the pancreatic beta cells to secrete insulin can be evaluated by directly measuring either insulin or C-peptide.   In some instances, C-peptide levels more accurately reflect islet cell function. 

    Increased levels indicate:

    • Insulinoma
    • Pancreas transplant
    • Renal failure
    • Administration of oral hypoglycemic agents

    Decreased levels indicate:

    • Factitious hypoglycemia
    • Diabetes mellitus
    • Total pancreatectomy
    Related Tests
    • Glucose, Blood.
    • Glucagon - an islet cell hormone that acts to increase serum glucose levels.
    • Glycosylated hemoglobin - an indirect measure of the chronic state of glucose levels.
    • Insulin assay - direct measurement of insulin.
    Drug-Lab Interactions
    • Renal failure can cause increased levels of C-peptide.
    • Drugs that may cause increased levels of C-peptide include oral hypoglycemic agents (e.g., sulfonylureas).
    Test Tube Needed
    • Marbled or red top tube
    • Collect a venous blood sample.
    • Apply pressure or a pressure dressing to the venipuncture site and assess the site for bleeding.
    Storage and Handling
    • Store sample at -70°C; serum is stable for 30 days when frozen.
    What To Tell Patient Before & After
    • Explain the procedure to the patient.
    • Instruct the patient to fast for 8 to 10 hours before the test.  Only water is permitted.
    • LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.
    • Pagana K, Pagana TJ eds. Mosby's Manual of Diagnostic and Laboratory Tests. 5th Ed.  St. Louis, Missouri. 2014

MESH Terms & Keywords

  • C, Peptide, Insulin C