Abstract: Methods to determine the concentrations in a sample of proinsulin and of molecules comprising C-peptide are disclosed. The concentrations are determined within the same reaction compartment and using only one capture antibody, specific for an epitope within C-peptide. Kits are disclosed for use in the disclosed methods. Methods to determine the concentrations in a sample of n analytes from a group of N analytes and the total concentration of the N analytes are disclosed. N is 2 or more and n is any integer value from 1 to N. Each of the N analytes has at least one common portion present in each other analyte, and the n analytes have each a distinct portion not present in any other of the N analytes. The concentrations are determined within the same reaction compartment using only one capture antibody which is specific for an epitope within the common portion.