APCS and Alzheimer disease: Although many studies have identified plasma proteins related to AD [e.g., BDNF (Laske et al., 2007; Laske et al., 2011)], complement C4a (Bennett et al., 2012), IGFBP-2 (McLimans et al., 2017), TTR (Uchida et al., 2015), and SAP (Wilson et al., 2008), these are unlikely to be useful as a diagnostic test when used as single markers due to a lack of sensitivity and specificity.