Reducing interference from heterophilic antibodies in a two-site immunoassay for carcinoembryonic antigen (CEA) by using a human/mouse chimeric antibody to CEA as the tracer

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To reduce heterophilic antibody interference in a two-site immunoassay for carcinoembryonic antigen (CEA), we utilized a human/mouse chimeric antibody to CEA as the tracer. One mouse monoclonal antibody (MAb), F82-61, which reacts with an epitope present on the domain N of CEA, was immobilized on 96-well polystyrene microtiter plates. A human/mouse chimeric antibody (Ch F11-39), which recognizes an epitope present on the domain B3 of CEA, was biotinylated for the tracer (Ch F11-39 system). Another MAb F11-39, the parental MAb of Ch F11-39, was also biotinylated and used as the control tracer (F11-39 system). For a fair comparison, the same 503 serum samples from healthy individuals were simultaneously assayed in the present study. When a tentative common reference limit of 5 ng/ml was used, the false positive rate with the Ch F11-39 system was only 2.8% ( 14 503 ) and that with the F11-39 system was 29.0% ( 146 503 ). Adding normal mouse serum (NMS; 1%) or a mixture of purified mouse IgG subclasses (heterophilic blocking reagent (HBR, 15 μg/test)) to the F11-39 system reduced the false positive rate from 29.0% to 6.2% ( 31 503 ) or 4.8% ( 24 503 ), respectively, suggesting that heterophilic antibodies reactive with mouse IgG gave rise to the high positive rate in normal populations with the F11-39 system. On the other hand, the false positive rate with the Ch F11-39 system was only slightly reduced from 2.8% to 2.6% ( 13 503 ) or to 2.0% ( 10 503 ) by adding MMS or HBR to the Ch F11-39 system. The false positive rates with two commercially available assay systems, CEA Roche EIA · DM or Abbott IMx CEA, were 5.4% ( 27 503 ) and 5.8% ( 29 503 ), respectively, which both corresponded roughly to that with the F11-39 system including NMS or HBR. These results indicate that the application of human/mouse chimeric antibodies in two-site immunoassays is more effective for reducing interference from heterophilic antibodies than the adding of NMS or purified mouse IgG in the assay using conventional MAbs.