OBJECTIVE: We examined agreement and disagreement between 2 biomarkers of 
β-amyloid (Aβ) deposition (amyloid positron emission tomography [PET] and 
cerebrospinal fluid [CSF] Aβ1-42 ) in normal aging and dementia in a large 
multicenter study.
METHODS: Concurrently acquired florbetapir PET and CSF Aβ were measured in 
cognitively normal, mild cognitive impairment (MCI), and Alzheimer's disease 
participants (n = 374) from the Alzheimer's Disease Neuroimaging Initiative. We 
also compared Aβ measurements in a separate group with serial CSF measurements 
over 3.1 ± 0.8 years that preceded a single florbetapir session. Additional 
biomarker and cognitive data allowed us to further examine profiles of 
discordant cases.
RESULTS: Florbetapir and CSF Aβ were inversely correlated across all diagnostic 
groups, and dichotomous measurements were in agreement in 86% of subjects. Among 
subjects showing the most disagreement, the 2 discordant groups had different 
profiles: the florbetapir(+) /CSF Aβ(-) group was larger (n = 13) and was made 
up of only normal and early MCI subjects, whereas the florbetapir(-) /CSF Aβ(+) 
group was smaller (n = 7) and had poorer cognitive function and higher CSF tau, 
but no ApoE4 carriers. In the longitudinal sample, we observed both stable 
longitudinal CSF Aβ trajectories and those actively transitioning from normal to 
abnormal, but the final CSF Aβ measurements were in good agreement with 
florbetapir cortical retention.
INTERPRETATION: CSF and amyloid PET measurements of Aβ were consistent in the 
majority of subjects in the cross-sectional and longitudinal populations. Based 
on our analysis of discordant subjects, the available evidence did not show that 
CSF Aβ regularly becomes abnormal prior to fibrillar Aβ accumulation early in 
the course of disease.