C-peptide test is a blood test which is carried out to find out how much insulin your body is producing.

C-peptide test is done for:

• Distinguishing whether you have type 1 or type 2 diabetes or MODY
• To investigate whether you have insulin resistance

The connecting peptide, or C-peptide, is a short 31-amino-acid polypeptide that connects insulin’s A-chain to its B-chain in the proinsulin molecule. After cleavage of proinsulin in the pancreatic β-cells, C-peptide is secreted into the portal circulation in equimolar concentrations with insulin. For each molecule of insulin produced there is a molecule of C-peptide. C-peptide is a useful marker of insulin production because C-peptide tends to remain in the blood longer than insulin.

A normal C-peptide range is 0.5 to 2.72 ng/ml (0.17 to 0.9 nmol/L).

C-peptide levels taken within the first few years of diagnosis may be useful in confirming Type 1 diabetes if results are low (e.g. non-fasting blood C-peptide < 0.2 nmol/l with hyperglycaemia confirms severe insulin deficiency, < 0.6 nmol/l Type 1 diabetes likely). C-peptide levels are elevated in insulinoma, sulfonylurea intoxication, insulin resistance state etc.

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