Sandwich ELISA for the measurement of Apo-E4 levels in serum and the estimation of the allelic status of Apo-E4 isoforms.

Apolipoprotein-E (Apo-E) isoforms, especially theApo-E4 isoform, are considered to be genetic risk factors for familial as well as sporadic Alzheimer's disease (AD). Using monoclonal antibodies specific for Apo-E4 (MAb412-1-7) and Apo-E (LZ18), we established a sandwich ELISA for the measurement of Apo-E4 levels in serum. Total Apo-E also was determined by sandwich ELISA with a second pair of monoclonal antibodies, LZ12 and LZ18. Estimation of a patient's homozygous or heterozygous status for the Apo-E4 isoform was subsequently calculated as the ratio of Apo-E4/total Apo-E, respectively. Although the antibody for Apo-E used here does not have equivalent reactivity toward Apo-E2, -E3, or -E4, we were able to attain a fairly reliable assay system to estimate the status of the Apo-E4 isoform through a simple formula for the calculation of total Apo-E versus Apo-E4 isoform.