AbstractBackgroundAmyloid-β PET and CSF Aβ₄₂yield discordant results in 10–20% of memory clinic patients, possibly providing unique information. Although the predictive power of demographic, clinical, genetic, and imaging features for amyloid positivity has previously been investigated, it is unknown whether these features differentially predict amyloid-β status based on PET or CSF or whether this differs by disease stage.MethodsWe included 768 patients (subjective cognitive decline (SCD,n = 194), mild cognitive impairment (MCI,n = 127), dementia (AD and non-AD,n = 447) with amyloid-β PET and CSF Aβ₄₂measurement within 1 year. Ninety-seven (13%) patients had discordant PET/CSF amyloid-β status. We performed parallel random forest models predicting separately PET and CSF status using 17 patient features (demographics, APOE4 positivity, CSF (p)tau, cognitive performance, and MRI visual ratings) in the total patient group and stratified by syndrome diagnosis. Thereafter, we selected features with the highest variable importance measure (VIM) as input for logistic regression models, where amyloid status on either PET or CSF was predicted by (i) the selected patient feature and (ii) the patient feature adjusted for the status of the other amyloid modality.ResultsAPOE4, CSF tau, and p-tau had the highest VIM for PET and CSF in all groups. In the amyloid-adjusted logistic regression models, p-tau was a significant predictor for PET-amyloid in SCD (OR = 1.02 [1.01–1.04],p_FDR = 0.03), MCI (OR = 1.05 [1.02–1.07],p_FDR < 0.01), and dementia (OR = 1.04 [1.03–1.05],p_FDR < 0.001), but not for CSF-amyloid. APOE4 (OR = 3.07 [1.33–7.07],p_unc < 0.01) was associated with CSF-amyloid in SCD, while it was only predictive for PET-amyloid in MCI (OR = 9.44 [2.93, 30.39],p_FDR < 0.01). Worse MMSE scores (OR = 1.21 [1.03–1.41],p_unc = 0.02) were associated to CSF-amyloid status in SCD, whereas worse memory (OR = 1.17 [1.05–1.31],p_FDR = 0.02) only predicted PET positivity in dementia.ConclusionAmyloid status based on either PET or CSF was predicted by different patient features, and this varied by disease stage, suggesting that PET-CSF discordance yields unique information. The stronger associations of both APOE4 carriership and worse memory z-scores with CSF-amyloid in SCD suggest that CSF-amyloid is more sensitive early in the disease course. The higher predictive value of CSF p-tau for a positive PET scan suggests that PET is more specific to AD pathology.