Normally, measuring insulin directly is more accurate with diabetics. But C-peptide levels more accurately reflect islet cell function in situations of insulinomas as well as cases of diabetics taking exogenous insulin (for treatment or secretly).
C-peptide, short for "connecting peptide" is the protein connecting beta/alpha chains of proinsulin. The chains are separated when proinsulin becomes insulin and C-peptide. C-peptide ends up in equal amounts to insulin in the portal vein, lasts longer than insulin so can be found more readily in peripheral circulation, and correlates with insulin levels.
Pagana, K.D., Pagana, T.J. Mostby's Manual of Diagnostic and Laboratory Tests, 3rd ed. Mosby Elsvier, 2006, p. 197.