Interference with C-peptide Assay by Anti-murine Antibodies

Abstract

We report interference with C-peptide assay by and-murine antibodies observed in two diabetic patients. The Daiichi C-peptide kit III (method 1), a two-antibody method, uses mouse monoclonal antibody (mmab) for the second antibody. Its use in Japan exceeded 50% in 1995. When determined by method 1, CPR values of our patients were high compared with the values of IRI. Fasting values of IRI and CPR were 2μU/ml and 3.3 ng/ml, respectively in case 1. These values were 5 and 9.5, respectively, in case 2, although no interference was observed in samples obtained 1.8 years earlier. Neither patient had detectable antibodies against insulin or C-peptide. Their proinsulin levels were not increased. When the same samples were assayed by Shionogi C-peptide RIA (method 2) which does not use mmab, CPR values were reduced to 0.7 ng/ml in both patients. The slope of the linear regression lines between CPR values determined by these two methods seems to reflect the degree of interference. Treatment of sera with polyethyleneglycol or protein A eliminated the interference in method 1. When the second antibody in method 1 was changed to rabbit polyclonal antibody, CPR values became close to those obtained by method 2. Addition of mouse IgG also decreased interference. We conclude that anti-murine antibodies in serum interfere with the second antibody reaction in method 1, resulting in abnormally high CPR values.