C-Peptide suppression test

Last updated: Thursday, 08, April, 2004

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Item Process

5 mL blood in EDTA tube plus 5 mL blood in lithium heparin or fluoride/oxalate tube.


Glucose: spectrophotometry
C-peptide: immunoassay.


After an overnight fast, the patient is given an infusion of insulin (0.125 U/kg) over 60 minutes.

Specimens are collected immediately prior to the infusion, then every 30 minutes for two hours.

Glucose and C-peptide analyses are performed.

The test must be performed with continuous medical monitoring, and may need to be terminated if significant clinical hypoglycaemia occurs.

Reference Interval

If glucose levels fall to <2.2 mmol/L, trough C-peptide should be <0.4 nmol/L (1.2 µg/L).


Diagnosis of insulinoma, though now largely replaced by other protocols.


If C-peptide does not fall to within the reference interval in a patient who becomes hypoglycaemic the patient has an autonomous source of endogenous insulin.

A decrease in the percentage fall from the baseline value may also be used as a diagnostic criterion as the occasional patient with an insulinoma may have a low baseline level.


Service FJ et al. J Clin Endocrinol Metab 1992; 74: 204-210.

Grant CS. Baillieres Clin Gastroenterol 1996; 10(4): 645-671.