Serum as an alternative to plasma for determining the ApoE4 phenotype using Pan-ApoE and ApoE4 chemiluminescent enzyme immunoassays

Objectives Since apolipoprotein E4 (ApoE4) is associated with therapy-related adverse effects of anti-amyloid-β monoclonal antibodies in Alzheimer’s disease, such as amyloid-related imaging abnormalities, methods for determining the ApoE4 status are needed. The Lumipulse ® ApoE4 and Pan-ApoE assays provide a practical approach, but they have so far been validated only in plasma. Methods Plasma and serum obtained from a single blood draw from 157 patients were analyzed using ApoE4 and Pan-ApoE chemiluminescent enzyme immunoassays. The APOE genotype was determined by allele-specific quantitative polymerase chain reaction. Additional experiments assessed assay repeatability, intermediate precision, and sample stability under various storage conditions. Results ApoE4, Pan-ApoE, and the resulting ApoE4/Pan-ApoE ratio showed minimal bias, strong positive correlation and linearity between plasma and serum. Using matrix-specific thresholds, plasma- and serum-derived ApoE4/Pan-ApoE ratios accurately classified the ApoE4 phenotypes in concordance with APOE ε4 genotypes. The assay demonstrated repeatability of <15 % and <5 % (day 1 and 2), with intermediate precision <10 % for both Pan-ApoE and ApoE4. Plasma and serum showed both comparable stability after multiple freeze-thaw-cycles and up to 120 h at 4 °C, while serum showed deterioration after 120 h storage at 20 °C. Conclusions Using matrix-specific thresholds, the ApoE4/Pan-ApoE ratio in plasma and serum accurately classified the ApoE4 phenotype in concordance with APOE ε4 genotypes. Serum and plasma showed comparable stability and precision. Thus, serum is a reliable alternative to plasma for measuring Pan-ApoE and ApoE4 with the Lumipulse assays. The use of serum expands sample availability, particularly in retrospective studies or when collected alongside cerebrospinal fluid.