1) The document describes a trefoil type multiplex immunoassay that can quantify different peptide molecules of the same family within a single assay. 2) It applies this method to simultaneously measure proinsulin and C-peptide levels to determine their ratio (PI/C ratio), an important marker for pancreatic beta cell function. 3) The assay uses three antibodies - one common capture antibody, one common detection antibody, and molecule-specific labeled antibodies - to independently measure the levels of each molecule within the same reaction compartment.