C-peptide comparative radioimmunoassays: a study of three commercial kits

Plasma C-peptide immunoreactivity (CPR) was measured in 18 fasting subjects with three different commercial kits (RIA-mat C-peptide, Byk-Mallinckrodt : RIA-gnost - h C-peptide, Hoechst-Behring : human C-peptide radioimmunoassay kit, Novo). The subjects were chosen as to cover a wide range of CPR concentrations (five healthy subjects, six obese subjects, three insulin-dependent diabetics, four normal subjects whose plasmas had been kept at - 20 degrees C for periods of 16 or 36 months). CPR was measured with the Novo kit in eight other plasmas which were kept over a period of 36 months, with or without aprotinin. Good correlations have been established among the values found with the three kits. However, absolute concentration values for each subject as well as the dispersion of all plasma C-peptide values varied as a function of the kit used because of antibody specificity differences and because of the various separation methods. The normal range proposed changes with each kit and the blood CPR of a subject can be a normal, reduced or increased one, depending on the kit used. After several months of storage, plasma CPR degradation is observed with the three kits. A protease-inhibitor is necessary in order to avoid this C-peptide degradation due to the apparent existence of a plasma proteolytic enzyme.