This study also helps to explain the improved agreement between amyloid PET and CSF measures observed with the Aβ1‐42/Aβ1‐40 ratio in non‐AD dementias.6, 7 Because both Aβ1‐40 and Aβ1‐38 are reduced in FTLD but not in AD, the Aβ1‐42/Aβ1‐40 and Aβ1‐42/Aβ1‐38 ratios can correct for the effect of FTLD‐related neurodegeneration on the CSF levels of APP‐derived peptides yielding a measure that is more specifically linked to cerebral amyloid deposition than levels of Aβ1‐42 alone. Here, APP is linked to Alzheimer disease.