C-peptide

A fragment released in equal amounts to insulin when the pancreas makes it, but cleared far more slowly.

Because injected insulin contains no C-peptide, it is what separates insulin your own pancreas made from insulin you were given.

It lasts much longer in the blood than insulin does, which makes it the steadier of the two to chart. Kidney disease raises it without any change in what the pancreas is doing.

Worth discussing a value outside the range together with fasting glucose.