Alzheimer's Imaging Consortium

Ten years after the original publication, the Centiloid framework is now broadly used to harmonize amyloid-PET quantification, facilitate data sharing and comparison across cohorts, and even assist visual interpretation in clinical settings. We evaluated the global implementation of Centiloids by comparing their distribution and corresponding positivity thresholds across cohorts.

We gathered data from publicly available cohorts and reached out to investigators across the world to collect cross-sectional Centiloids, demographic and clinical information, and visual read data. Gaussian mixture models (GMM, k=2) were fitted to Centiloid values for each cohort and cutoffs were calculated as mean + 2SD of the lower Gaussian. When visual reads were available, we determined Centiloid cutoffs that maximized correspondence with visual reads (Cohen's kappa). Data was combined across cohorts using random effects meta-analyses.

As of January 2025, we included 37 cohorts (n = 41,678 participants) with heterogeneous pipelines, radiotracers, and clinical and demographic characteristics (Table-1). The low Gaussian peaks ranged from -9 to 10CL; the meta-analysis identified a common peak at 1CL. The second peaks were more heterogeneous (range=38-102CL; meta-analysis outcome=64CL). Across cohorts, the proportion of cognitively unimpaired versus impaired participants impacted the position of both peaks, with better separation in cohorts enriched in impaired individuals (Figure-1C). The meta-analysis indicated a GMM-based cutoff of 19CL (95%CI: 16-21CL, Figure-1B); subgroup analyses showed no evidence of significant effect between single versus multicenter settings (17 versus 20CL, p = 0.30), MRI-based or PET-only processing (18 versus 19CL, p = 0.88), and no evidence of difference across radiotracers (Flutemetamol: 16CL; PIB: 17CL, Flutafuranol: 18CL, Florbetaben: 19CL, Florbetapir: 20CL, p = 0.78). In a subset of 29,496 participants with visual reads available, binary visual reads corresponded well to Centiloids (common kappa=0.86, Figure-2A). The visual read-based cutoff of 24CL (95%CI: 21-27CL, Figure-2B) maximized correspondence between visual read and quantification and was slightly higher than the GMM-based cutoff. All meta-analysis models showed high non-random heterogeneity (I²>80%) across studies, suggesting non-random differences in peaks and cutoffs.

Meta-analysis-based cutoffs align well with thresholds from the existing literature. High heterogeneity among studies underscores the need to investigate contributing factors, raising concerns about applying common cutoffs.