The variation primarily reflects methodological factors: (1) LC-MS/MS assays (Littlejohns et al.)reduce measurement error vs. immunoassays (Schneider et al.), with meta-regression confirming a 0.33 lower log-RR for LC-MS/MS (p = 0.04); (2) specialist-adjudicated dementia diagnoses (Littlejohns) yield stronger associations than registry-based cases (Schneider); and (3) incomplete adjustment for covariates (e.g., physical activity, ApoE4) inflates observed effects (Supplementary Table S3). Here, APOE is linked to dementia.