sAPP heteromers contribute to the estimation of sAPPα and sAPPβ levels by ELISA. (A) Box plot of CSF levels of classical AD biomarkers Aβ42, T-tau and P-tau from 13 NDC controls and 13 probable AD cases. (B) CSF samples were also assayed for specific sAPPα and sAPPβ using ELISA kits from IBL before (incubated with Sepharose A beads in the absence of an antibody) and after immunoprecipitation (IP) with the Sigma-Ct antibody. Before immunoprecipitation, no differences were found between NDC and AD subjects. After immunoprecipitation, levels of sAPPα and sAPPβ reduced significantly in both groups (paired Student-t test). (C) 10 fresh aliquots (from the 13 cases available) from NDC and AD groups were assayed with an alternative ELISA kit from Novex, of which the detection antibody binds to an epitope present in sAPPα and absent in sAPPβ, thus recognizing APPα and sAPPf. Before immunoprecipitation, no differences were found between NDC and AD subjects. Immunoprecipitation significantly reduced the levels of sAPPα and sAPPf in NDC and AD subjects (paired Student-t test). The data represent the means ± SEM (n.s., not significant).