[IgG heterophile antibody causes false positivity for CA19-9, which is overcome with bovine immunoglobulin]

Immunoassay using antibodies is widely applied to measure various clinical parameters such as tumor markers. However, many mechanisms of interference for this method have been reported. We studied serum sample showing false positive CA19-9 values in the range of 24 to approximately 286U/ml depending on the reagent lots used. PEG-pretreated serum did not show false positivity. Using high performance liquid chromatography (HPLC), the CA19-9 false positive peak was obtained around 160kD, which is the same as IgG. Antibody-coated microbeads, labeled antibody and buffer were used in different combinations between different lots, and the buffer was found to be the cause of false positive findings. The amount of bovine IgG contaminated in bovine serum albumin(BSA) in the reagent buffer markedly differed between lots of BSA, and reagent with a low amount of bovine IgG showed false positive results. Bovine immunoglobulin was superior to mouse IgG in the attempt to avoid a false positive reaction. We concluded that the cause of false positive CA19-9 findings in this serum sample was IgG heterophile antibody, which reacts with both of mouse and bovine IgG. The heterophile antibody had greater affinity to bovine IgG than to mouse IgG. The difference between CA19-9 values obtained by different reagent lots was due to different amounts of bovine IgG contaminated in BSA used in the reagent. Bovine immunoglobulin was superior to mouse IgG in the attempt to absorb the heterophile antibody and avoid false positive reaction in this case.