Alzheimer's disease (AD) can only be definitively diagnosed with autopsy findings; in the absence of autopsy, clinical diagnoses of AD are "possible" or "probable", based on other findings.[108][109][110] Up to 23% of those clinically diagnosed with AD may be misdiagnosed and may have pathology suggestive of another condition with symptoms that mimic those of AD.[109]

AD is usually clinically diagnosed based on the person's medical history, history from relatives, and behavioral observations. The presence of characteristic neurological and neuropsychological features and the absence of alternative conditions supports the diagnosis.[needs update][111][112] Advanced medical imaging with computed tomography (CT) or magnetic resonance imaging (MRI), and with single-photon emission computed tomography (SPECT) or positron emission tomography (PET), can be used to help exclude other cerebral pathology or subtypes of dementia.[113] Moreover, it may predict conversion from prodromal stages (mild cognitive impairment) to Alzheimer's disease.[114] FDA-approved radiopharmaceutical diagnostic agents used in PET for Alzheimer's disease are florbetapir (2012), flutemetamol (2013), florbetaben (2014), and flortaucipir (2020).[115] Because many insurance companies in the United States do not cover this procedure, its use in clinical practice is largely limited to clinical trials as of 2018.[116]

Assessment of intellectual functioning including memory testing can further characterise the state of the disease.[1] Medical organizations have created diagnostic criteria to ease and standardise the diagnostic process for practising physicians. Definitive diagnosis can only be confirmed with post-mortem evaluations when brain material is available and can be examined histologically for senile plaques and neurofibrillary tangles.[116][117]