CST3 and Alzheimer disease: Cystatin C colocalises with amyloid and is involved in microglial activation.31 Several previous biomarker discovery studies have compared concentrations of cystatin C in AD and control CSF using ELISA, sometimes with equivocal or conflicting results.32, 33, 34 Cystatin C has also been identified using mass spectrometry in biomarker discovery studies of AD CSF.35, 36 Our findings replicate these results in two further independent cohorts, suggesting that mass spectrometry may be a more sensitive and reproducible method for quantifying this protein.