66 Chapter 3 Table 2. Cut-off values for different methods Derivation method Threshold N positive (%) Kappa (95% CI) Visual assessment 38 (24%) BPND Low K-means 0.19 41 (25%) 0.65 (0.51-0.79) GMM 0.23 30 (19%) 0.70 (0.57-0.84) High K-means 0.29 26 (16%) 0.65 (0.51-0.80) SUVr Low K-means 1.28 37 (23%) 0.60 (0.45-0.75) GMM 1.34 25 (15%) 0.63 (0.48-0.78) High K-means 1.43 22 (14%) 0.60 (0.44-0.75) Cohen’s kappa was used to determine the degree of concordance between visual assessment and the six different thresholds. BPND= Binding potential, GMM = Gaussian mixture modelling, SUVr= standardized uptake value ratio. Amyloid positivity thresholds in relation to memory slopes We investigated the association between different definitions of amyloid positivity and memory slopes. We found each operationalization of amyloid positivity was associated with rate of decline on the RAVLT delayed recall (Table 3). Models in which A+ was defined by visual assessment or BPND thresholds performed somewhat better than models based on SUVr (i.e. lower AIC values).
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