Amyloid density (after square root transformation) is based on historical and/or digital data and is the mean of the square root transformation of the amyloid score in 8 regions:

1. Angular gyrus
2. Anterior cingulate
3. Calcarine cortex
4. Entorhinal cortex
5. Hippocampus
6. Inferior temporal
7. Midfrontal gyrus
8. Superior frontal

Historical measures of β-amyloid (Aβ) were obtained from 20-micron sections from 8 brain regions. Immunohistochemistry was performed using 3 monoclonal antibodies against Aβ, 4G8 (1:9000; Covance Labs, Madison, WI), 6F/3D (1:50; Dako North America, Inc., Carpinteria, CA), and 10D5 (1:600; Elan Pharmaceuticals, San Francisco, CA). Cortical and hippocampal regions of interest were manually outlined (excluding meninges) using Sterio Investigator software. The systematic random sampling (SRS) grid within the software was applied to the regions of interest. Images at each sampling grid were manually taken, and the percentage of images positive for amyloid was calculated.

Digital scanning of 4G8-stained slides began in 2019 using the Aperio AT2 whole slide brightfield scanning system. Briefly, paraformaldehyde-fixed brain regions are sampled, embedded, sectioned at 6-microns, and subsequently stained using a 4G8 antibody (1:9000; Covance Labs, Madison, WI). Pathology glass slides are loaded into the Aperio AT2 scanner console and processed to generate digital whole slide images. Cortical and hippocampal regions of interest were manually outlined (excluding meninges). Digital image analysis for Aβ pathology was quantified using the Aperio Image Analysis Toolbox Positive Pixel Count algorithm. Algorithm input parameters were optimized for Aβ staining intensity and applied to whole slide images from the 8 brain regions. Digital scores are the percentage of amyloid (after square root transformation) and calculated for each individual slide.

This variable harmonizes historical data with the new digital data for Aβ. The variable uses the new digital scores for Aβ. If digital data is not available for a particular region, a digital score is predicted from the historical data. The mean of the 8 region values is calculated if data for at least 4 of the 8 regions are available.

This variable replaces "amyloid" as the measure of percent of β-amyloid positivity in the cortex.