We tested CSF TREM2 in subjects with preclinical AD, mild cognitive impairment (MCI) due to AD (MCI‐AD), and AD dementia and controls, defined by clinical and CSF biomarker criteria as recommended by the National Institute on Aging‐Alzheimer's Association (NIA‐AA) criteria (Albert et al, 2011; McKhann et al, 2011; Sperling et al, 2011). Here, TREM2 is linked to Alzheimer disease.