C-Peptide suppression test
Last updated: Thursday, 08, April, 2004
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| Item | Process |
|---|---|
| Specimen | 5 mL blood in EDTA tube plus 5 mL blood in lithium heparin or fluoride/oxalate tube. |
| Method | Glucose: spectrophotometry |
| 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(4): 645-671. |