CST3 and Alzheimer disease: In contrast, a previous study reported that the plasma cystatin C levels were lower in AD patients than in controls (Chuo et al., 2007), and another studies did not find significant differences in plasma cystatin C levels between AD patients and healthy controls (Kálmán et al., 2000; Ghidoni et al., 2010; Zhong et al., 2013).