CLU and Alzheimer disease: Apathy has been observed in some cohorts to be the most prevalent NPS in AD, with a point prevalence of 49%.165 Positive correlations between apathy and plasma levels of γ-aminobutyric acid (GABA), a key inhibitory neurotransmitter in the central nervous system, have been observed.166 The presence of apathy has also been associated with a higher plasma/platelet clusterin ratio in AD.167 However, evidence for the association between plasma homocysteine levels and apathy has been mixed.