C Peptide

This blood test is for C-peptide, a substance produced by the pancreas in equal proportions to insulin. It is a more stable compound than insulin, so practically it can be used to assess insulin secretion, in both normal subjects and in those who are insulin treated. Low C-peptide levels indicate decreased insulin production.

C-peptide levels can also help to differentiate between type 1 and type 2 diabetes. In type 1 diabetes, the pancreas produces little or no insulin, and therefore little or no C-peptide. In type 2 diabetes, C-peptide levels will typically be normal or high, as the pancreas is working harder to overcome insulin resistance (reduced sensitivity to the effects of insulin) by producing more insulin. An insulinoma (tumour producing insulin) will also give rise to high C-peptide levels.

Often requested alongside a test for Fasting Glucose, a low level of C peptide with a normal or high Fasting Glucose level may indicate Type 1 diabetes, or longstanding Type 2 diabetes. If the Fasting Glucose level is low and the C peptide is also low, it may just indicate a lack of food intake recently or rarely Addison's disease.