C-peptide
C-peptide is a byproduct to the body´s own production of insulin and is produced 1:1 to endogenous insulin (1). It can be measured in blood and urine and is often used as a diagnose tool, as well as following the progress of diabetes in an individual. It is also used to differentiate forms of diabetes from each other, for example MODY and type 2 diabetes (see more below). C-peptide has long been thought to lack function in the body but last years this have been questioned. Still we don´t know. There was a study in Sweden a few years ago that wanted to see if injected C-peptide had impact on neuropathy, but the study failed to show any positive effect.
It´s measured as a fasting value and sometimes stimulated as an OGTT (oral glucose tolerance test) or MMTT (mixed meal tolerance test). Half-life of C-peptide is 20-30 min compared to insulin of 3-5 min, as well as insulin is cleared differently and C-peptide more consistent. This makes a test of C-peptide more reliable and stabile than measuring insulin. The insulin people with diabetes inject doesn´t contain C-peptide so there is no risk for conflicting result. There is no exact international consensus for a reference range, but about 0.8-3.1 ng/mL (conventional units), or 0.26-1.03 nmol/L is fasting value for someone without diabetes (2), and postprandial (after a meal) 3-9 ng/mL or 1-3 nmol/L (3).
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