C-Peptide
Test Description:
Proinsulin is converted to insulin in the beta-cells of the pancreas. A by-product of
this conversion is C-peptide, an inactive amino acid. C-peptide levels usually correlate
with endogenous insulin levels and are not affected by exogenous insulin
administration. Measurement of C-peptide levels is useful in:
• determining endogenous insulin levels, since the C-peptide level is not affected
by exogenous insulin administration;
• determining whether hypoglycemia is being caused by nontherapeutic insulin
injections (factitious hypoglycemia), in which case the C-peptide level will be low
while insulin levels are elevated;
• determining presence of an insulinoma, which can be indicated by elevation of
both insulin and C-peptide levels; and
• monitoring for recurrence of insulinoma, indicated by increasing C-peptide levels.
Normal Values
0.5–2.0 ng/mL (0.17–0.67 nmol/L SI units)
Possible Meanings of Abnormal Values
Increased Decreased
Insulinoma Diabetes mellitus
Islet cell tumor Hypoglycemia due to insulin
Pancreas transplants insulin overdose/abuse
Renal failure Pancreatectomy
Contributing Factors to Abnormal Values
• Hemolysis of the blood sample may alter test results.
• C-peptide levels may not correlate with endogenous insulin levels in the presence of
obesity or islet cell tumors.
• Drugs which may increase C-peptide levels: sulfonylureas.
Interventions/Implications
Pretest
• Explain to the patient the purpose of the test and the need for a blood sample to be drawn.
• Fasting for 8 to 10 hours is required prior to the test. Water is permitted.
Procedure
• A 7-mL blood sample is drawn in a red-top collection tube.
• Gloves are worn throughout the procedure.
Posttest
• Apply pressure at venipuncture site. Apply dressing, periodically assessing for continued
bleeding.
• Label the specimen and transport it to the laboratory.
• Report abnormal findings to the primary care provider.
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