C-peptide specific assay procedure

A method for assaying C-peptide in a sample suspected of containing C-peptide, intact proinsulin, des-31,32 proinsulin, and/or des-64,65-proinsulin comprising:

• a) contacting the sample with at least a first anti-peptide antibody which specifically binds an epitope present entirely in the C-peptide and a second antibody which is selected from:
• an anti-proinsulin antibody which is specific to intact proinsulin and des-31,32-proinsulin and recognizes an epitope located in an intact A-chain-C-peptide (AC) junction of proinsulin and des-31,32 proinsulin, said anti-proinsulin AC junction antibody epitope being either adjacent to or overlapping with the epitope of said at least one anti-C-peptide antibody, or
• an anti-proinsulin antibody which is specific to intact proinsulin and des-64,65-proinsulin and recognizes an epitope located in an intact B-chain-C-peptide (BC) junction of proinsulin and des-64,65-proinsulin, said anti-proinsulin BC junction antibody epitope being either adjacent to or overlapping with the epitope of said at least one anti-C-peptide antibody, or
• a mixture of said anti-proinsulin AC junction antibody and said anti-proinsulin BC junction antibody under conditions such that said at least one anti-C-peptide antibody binds to C-peptide in the sample, binding of the anti-proinsulin AC junction antibody to any proinsulin and/or des-31,32-proinsulin interferes with binding of said at least one anti-C-peptide antibody to intact proinsulin and des-31,32-proinsulin in the sample, and/or binding of the anti-proinsulin BC junction antibody to any proinsulin and/or des-64,65-proinsulin interferes with binding of said at least one anti-C-peptide antibody to intact proinsulin and des-64,65-proinsulin in the sample;
• and
• b) detecting amount of C-peptide bound to said at least one anti-C-peptide antibody in said contacted sample as an assay of the amount of C-peptide in the sample.