The field of neurodegenerative dementias, particularly Alzheimer’s disease (AD), has been revolutionized by the development of imaging and cerebrospinal fluid biomarkers and is on the brink of a new development: emerging plasma biomarkers. Research now recognizes the relationship between the cognitive-behavioral syndromic diagnosis (ie, the illness) and the etiologic diagnosis (the disease) — and the need to consider each separately when developing a diagnostic formulation. The National Institute on Aging and Alzheimer’s Association Research Framework uses the amyloid, tau, and neurodegeneration system to define AD biologically in living patients. Here is an overview of the framework, which requires biomarker evidence of amyloid plaques (amyloid positivity) and neurofibrillary tangles (tau positivity), with evidence of neurodegeneration (neurodegeneration positivity) to support the diagnosis.
The Diagnostic Approach for Symptomatic Patients
The differential diagnosis in symptomatic patients with mild cognitive impairment (MCI), mild behavioral impairment, or dementia is broad and includes multiple neurodegenerative diseases (eg, AD, frontotemporal lobar degeneration, dementia with Lewy bodies, argyrophilic grain disease, hippocampal sclerosis); vascular ischemic brain injury (eg, stroke); tumors; infectious, inflammatory, paraneoplastic, or demyelinating diseases; trauma; hydrocephalus, toxic/metabolic insults; and other rare diseases. The patient’s clinical syndrome narrows the differential diagnosis.
Once the clinician has a prioritized differential diagnosis of the brain disease or condition that is probably causing or contributing to the patient’s signs and symptoms, they can then select appropriate assessments and tests, typically starting with a laboratory panel and brain MRI. Strong evidence backed by practice recommendations also supports the use of fluorodeoxyglucose PET as a marker of functional brain abnormalities associated with dementia. Although molecular biomarkers are typically considered at the later stage of the clinical workup, the anticipated future availability of plasma biomarkers will probably change the timing of molecular biomarker assessment in patients with suspected cognitive impairment owing to AD.
Molecular PET Biomarkers
Three PET tracers approved by the US Food and Drug Administration (FDA) for the detection of cerebral amyloid plaques have high sensitivity (89%-98%) and specificity (88%-100%) compared with autopsy, the gold standard diagnostic tool. However, these scans are costly and are not reimbursed by Medicare and Medicaid. Because all amyloid PET scans are covered by the Veterans Administration, this test is more readily accessible for patients receiving VA benefits.
The appropriate-use criteria developed by the Amyloid Imaging Task Force recommends amyloid PET for patients with persistent or progressive MCI or dementia. In such patients, a negative amyloid PET scan would strongly weigh against AD, supporting a differential diagnosis of other etiologies. Although a positive amyloid PET scan in patients with MCI or dementia indicates the presence of amyloid plaques, it does not necessarily confirm AD as the cause. Cerebral amyloid plaques may coexist with other pathologies and increase with age, even in cognitively normal individuals.
The IDEAS study looked at the clinical utility of amyloid PET in a real-world dementia specialist setting. In the study, dementia subspecialists documented their presumed etiologic diagnosis (and level of confidence) before and after amyloid PET. Of the 11,409 patients who completed the study, the etiologic diagnosis changed from AD to non-AD in just over 25% of cases and from non-AD to AD in 10.5%. Clinical management changed in about 60% of patients with MCI and 63.5% of patients with dementia.
In May 2020, the FDA approved flortaucipir F-18, the first diagnostic tau radiotracer for use with PET to estimate the density and distribution of aggregated tau neurofibrillary tangles in adults with cognitive impairment undergoing evaluation for AD. Regulatory approval of flortaucipir F-18 was based on findings from two clinical trials of terminally ill patients who were followed to autopsy. The studies included patients with a spectrum of clinically diagnosed dementias and those with normal cognition. The primary outcome of the studies was accurate visual interpretation of the images in detecting advanced AD tau neurofibrillary tangle pathology (Braak stage V or VI tau pathology). Sensitivity of five trained readers ranged from 68% to 86%, and specificity ranged from 63% to 100%; interrater agreement was 0.87. Tau PET is not yet reimbursed and is therefore not yet readily available in the clinical setting. Moreover, appropriate use criteria have not yet been published.
Molecular Fluid Biomarkers
Cerebrospinal (CSF) fluid analysis is currently the most readily available and reimbursed test to aid in diagnosing AD, with appropriate-use criteria for patients with suspected AD. CSF biomarkers for AD are useful in cognitively impaired patients when the etiologic diagnosis is equivocal, there is only an intermediate level of diagnostic confidence, or there is very high confidence in the etiologic diagnosis. Testing for CSF biomarkers is also recommended for patients at very early clinical stages (eg, early MCI) or with atypical clinical presentations.
A decreased concentration of amyloid-beta 42 in CSF is a marker of amyloid neuritic plaques in the brain. An increased concentration of total tau in CSF reflects injury to neurons, and an increased concentration of specific isoforms of hyperphosphorylated tau reflects neurofibrillary tangles. Presently, the ratios of t-tau to amyloid-beta 42, amyloid-beta 42 to amyloid-beta 40, and phosphorylated-tau 181 to amyloid-beta 42 are the best-performing markers of AD neuropathologic changes and are more accurate than assessing individual biomarkers. These CSF biomarkers of AD have been validated against autopsy, and ratio values of CSF amyloid-beta 42 have been further validated against amyloid PET, with overall sensitivity and specificity of approximately 90% and 84%, respectively.
Some of the most exciting recent advances in AD center around the measurement of these proteins and others in plasma. Appropriate-use criteria for plasma biomarkers in the evaluation of patients with cognitive impairment were published in 2022. In addition to their use in clinical trials, these criteria cautiously recommend using these biomarkers in specialized memory clinics in the diagnostic workup of patients with cognitive symptoms, along with confirmatory CSF markers or PET. Additional data are needed before plasma biomarkers of AD are used as standalone diagnostic markers or considered in the primary care setting.
We have made remarkable progress toward more precise molecular diagnosis of brain diseases underlying cognitive impairment and dementia. Ongoing efforts to evaluate the utility of these measures in clinical practice include the need to increase diversity of patients and providers. Ultimately, the tremendous progress in molecular biomarkers for the diseases causing dementia will help the field work toward our common goal of early and accurate diagnosis, better management, and hope for people living with these diseases.
Bradford C. Dickerson, MD, MMSc
Professor, Department of Neurology, Harvard Medical School; Director, Frontotemporal Disorders Unit, Department of Neurology, Massachusetts General Hospital, Boston, Massachusetts
Disclosure: Bradford C. Dickerson, MD, MMSc, has disclosed no relevant financial relationships.
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