C-peptide suppression test

Keywords: C-peptide

Specimen:

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

Method:

Glucose: spectrophotometry

C peptide: immunoassay.

Protocol:

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).

Application:

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

Interpretation:

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.

Reference:

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

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