Plasma Proinsulin Assay (2)	

Whereas normal intracellular cleavage of proinsulin into insulin and C-peptide is usually almost complete and very little proinsulin enters the circulation, in most B-cell tumours, substantial amounts of intact proinsulin forms and partially-split forms (Fig. 20.15) are released into the circulation where they are detected as immunoreactive insulin. In some cases proinsulin and partially-split forms account for most, or even all, of the insulin immunoreactivity measurable in peripheral venous plasma. 

Insulinomas are usually associated with the highest and most inappropriate plasma immunoreactive insulin levels.