Could simple blood tests improve Alzheimer's diagnosis in the future?

Diagnosis of Alzheimer’s disease has relied on cognitive symptoms until recently. While brain imaging scans and cerebrospinal fluid assays can be used for Alzheimer’s diagnosis, these methods are expensive, invasive, or not easily accessible. Technological advances have finally led to the development of ultra-sensitive blood-based biomarkers that could allow early and inexpensive diagnosis and follow-up. While these blood-based biomarkers could transform routine clinical care and research, there are several challenges that need to be addressed before these biomarkers can be deployed in routine clinical care.

Alzheimer’s disease has been deemed a public health crisis, with an estimated 7 million individuals in the United States currently living with this neurodegenerative disease.

Alzheimer’s disease is a progressive disease, with gradual worsening of cognitive and functional abilities over time. Early diagnosis of AD can facilitate better management of the condition and delay its progression.

In the past four years, the Food and Drug Administration (FDA) approved the first three disease-modifying treatments, including aducanumab , lecanemab , and donanemab , which can delay or prevent the progression of Alzheimer’s.

These drugs are more effective in individuals in the early stages of Alzheimer’s disease, making its early diagnosis vital.

In addition, the advent of these therapies also necessitates the monitoring of treatment progression. This underscores the need for inexpensive, accessible, and accurate Alzheimer’s biomarkers.

Currently approved brain imaging and cerebrospinal fluid (CSF) biomarkers have high accuracy but are expensive and invasive. Blood-based biomarkers have been investigated as an accessible and cost-effective alternative to these biomarkers but have lacked accuracy.

Technological advances in the past decade have led to the development of ultrasensitive blood-based biomarker assays for Alzheimer’s disease.

These blood-based biomarkers could reduce the cost and time of recruitment for Alzheimer’s clinical trials and help monitor treatment outcomes. However, these biomarker assays still need to be standardized and validated before they can be used in primary care clinics.

Challenges with the clinical diagnosis of Alzheimer’s disease

Alzheimer’s disease is characterized by the abnormal accumulation and aggregation of the beta-amyloid and tau proteins. The aggregation of beta-amyloid protein leads to the formation of plaques between neurons, whereas the tau protein aggregates to form neurofibrillary tangles inside neurons.

The accumulation of beta-amyloid plaques in the brain precedes the emergence of cognitive symptoms by 1 to 3 decades. The period during which brain pathologies are present but cognitive symptoms have not manifested is referred to as the preclinical stage of Alzheimer’s disease.

In most cases, the diagnosis of Alzheimer’s disease is based on the presence of cognitive symptoms, such as memory loss. The diagnosis of Alzheimer’s disease based on such clinical symptoms is challenging even for experts in the field.

Studies suggest that around 25-30% of individuals with Alzheimer’s disease do not receive an appropriate diagnosis in specialized dementia clinics when relying on clinical symptoms alone.

Furthermore, monitoring disease progression and making a prognosis based solely on clinical symptoms is also challenging.

Limitations of conventional Alzheimer’s biomarkers

In 2018, the National Institute on Aging and the Alzheimer’s Association (NIA-AA) developed guidelines recommending a focus on pathological changes in the brain instead of clinical symptoms for Alzheimer’s diagnosis. In other words, Alzheimer’s can be diagnosed solely based on biological changes.

Specifically, this framework, also known as the ATN framework, recommends diagnosis of Alzheimer’s disease based on evidence of biomarkers for beta-amyloid deposits, tau neurofibrillary tangles, and neurodegeneration.

These pathological changes associated with Alzheimer’s disease are currently assessed using brain imaging scans or cerebrospinal fluid markers.

Positron emission tomography (PET) scans are used to assess the accumulation of beta-amyloid deposits and tau tangles, as well changes in glucose metabolism in the brain. Magnetic resonance imaging (MRI) scans are also sometimes used to detect structural changes in the brain.

The changes in glucose metabolism evaluated using PET and the decrease in the size of brain regions involved in cognition serve as biomarkers of neurodegeneration. Although accurate, brain imaging scans are expensive to conduct and require specialized facilities, thus limiting accessibility.

CSF, a fluid that bathes the brain, is also commonly used to assess the levels of amyloid and tau pathology and neurodegeneration. CSF is collected using a lumbar puncture , also known as a spinal tap. The invasiveness of this procedure and its perceived risk are limiting factors for the utilization of CSF biomarkers.

The high costs, invasiveness, and limited accessibility associated with brain imaging and cerebrospinal biomarkers make them unsuitable for Alzheimer’s diagnosis in routine clinical care.

These factors also pose a major challenge for the recruitment and follow-up of participants in large-scale clinical trials and population studies.

Blood-based biomarkers can be easily quantified in most clinical laboratories at a lower cost. However, until recently, the assays developed to measure biomarkers in the blood were not accurate.

The recent development of ultra-sensitive assays for these biomarkers holds promise to change the clinical and research landscape.

While these blood-based biomarkers are not yet ready to be used as a standalone tool in clinics, they are currently being used in clinical trials and soon in clinics in conjunction with cognitive assessments and/or brain imaging and CSF markers.

Suzanne Schindler, MD, PhD , an associate professor of neurology at Washington University in St. Louis, explained that: “Traditional tests for Alzheimer’s disease require a spinal tap or a specialized brain scan and are not readily available at many medical facilities. In contrast, most clinics routinely draw blood.  This means that blood tests for Alzheimer’s disease could be performed relatively easily, including by primary care providers.”

Why blood tests may be more convenient Alzheimer’s diagnostic tools

The recruitment of participants for Alzheimer’s disease clinical trials is a slow and expensive process. The cost of screening participants for Alzheimer’s disease clinical trials using PET and MRI scans is estimated to be $8,000 per participant.

Furthermore, about 44-88% of individuals fail the screening test for participation in Alzheimer’s disease clinical trials.

The use of blood-based biomarkers can reduce the cost and time needed to recruit clinical trial patients. Consistent with this, blood-based biomarkers have recently been used to screen clinical trial participants and measure treatment progress and outcomes.

These blood-based markers could also be used in a primary care setting to further assess the risk of Alzheimer’s disease in individuals showing deficits in cognitive assessments

Patients at an intermediate-to-high risk of Alzheimer’s, as evaluated using blood-based markers, could then be further assessed using imaging and cerebrospinal fluid biomarkers.

Screening older patients using blood biomarkers could also become a part of routine clinical assessments, enabling early identification of individuals with preclinical Alzheimer’s disease.

Identification of Alzheimer’s disease before the emergence of clinical symptoms could facilitate better management of the condition, especially given the availability of disease-modifying treatments.

Common blood-based biomarkers for Alzheimer’s

The initial attempts at the development of blood-based biomarkers for Alzheimer’s included the measurement of the beta-amyloid-42 protein found in plaques. However, these assays were not accurate .

Other studies focused on blood-based markers of neurodegeneration, such as neurofilament light and total tau , which are also present in other conditions besides Alzheimer’s disease.

Improvements in analytical methods have led to the development of assays that can detect biomarkers for amyloid and tau pathology and neurodegeneration.

These assays involve the quantification of the protein biomarkers with the help of antibodies or mass spectrometry.

Beta-amyloid levels

Beta-amyloid exists in several different forms, with beta-amyloid-40 and beta-amyloid-42 being the two prominent forms in the brain.

Beta-amyloid-42 contains 42 amino acid residues and is prone to aggregate and form plaques. Beta-amyloid-40, the shorter form of beta-amyloid that contains 40 amino acids, is less toxic and inhibits the aggregation of beta-amyloid-42.

The formation of amyloid plaques in Alzheimer’s disease is accompanied by a decrease in beta-amyloid-42 levels in the cerebrospinal fluid. The total levels of beta-amyloid-42, or the ratio of beta-amyloid-42 to beta-amyloid-40 in cerebrospinal fluid serve as Alzheimer’s disease biomarkers.

The ratio of beta-amyloid-42/beta-amyloid-40 normalizes the beta-amyloid-42 levels for inter-individual variation in the production of beta-amyloid. As a result, the beta-amyloid-42/ beta-amyloid-40 levels are a more accurate biomarker of amyloid pathology than beta-amyloid-42 concentrations alone.

The plasma shows a similar decline in the beta-amyloid-42/beta-amyloid-40 ratio to that observed in the cerebrospinal fluid and can be an accurate biomarker for beta-amyloid accumulation in the brain.

Individuals with a positive beta-amyloid PET scan only show an 8-15 % decline of beta-amyloid-42/beta-amyloid-40 ratio in the blood but a 40-60% decline in the cerebrospinal levels of beta-amyloid-42/ beta-amyloid-40.

The smaller decline in the beta-amyloid protein in the blood is likely due to its production in peripheral tissues instead of the brain. As a result, measuring plasma beta-amyloid-42/beta-amyloid-40 levels is more susceptible to errors, such as those during the processing of the blood samples or performing the assay.

Plasma tau biomarkers

Alzheimer’s disease involves the abnormal phosphorylation of the tau protein and its cleavage to form tau fragments that are prone to aggregation, forming neurofibrillary tangles. The phosphorylated tau (p-tau) fragments that do not aggregate are secreted into the cerebrospinal fluid and subsequently released into the bloodstream.

Several different variants or species of p-tau are present in the cerebrospinal fluid and plasma of individuals with Alzheimer’s disease. These variants differ in the site of phosphorylation.

Three p-tau species that have shown promise as Alzheimer’s disease biomarkers include p-tau181, p-tau217, and p-tau231, with the numerical representing the amino acid in the tau protein that is phosphorylated.

The levels of p-tau181 in the cerebrospinal fluid are used as an Alzheimer’s disease biomarker, whereas total tau in cerebrospinal fluid is an indicator of neurodegeneration.

Notably, the accumulation of amyloid deposits in the brain is thought to trigger the formation of these p-tau species. As a result, these p-tau species are observed only in Alzheimer’s disease but not in other neurodegenerative diseases involving tau accumulation.

Moreover, the levels of the aforementioned p-tau species are moderately correlated with the accumulation of neurofibrillary tangles in the brain. In contrast, levels of p-tau show a stronger correlation with the accumulation of amyloid-beta plaques in the brain.

Other p-tau species, such as p-tau202 and p-tau205, and microtubule-binding region (MTBR) of the tau protein are being investigated for their potential as better biomarkers for tau pathology.

The presence of the p-tau species precedes the appearance of clinical symptoms of Alzheimer’s disease as well as a positive beta-amyloid PET scan. Thus, these p-tau species could help identify individuals with preclinical Alzheimer’s disease.

In addition, the levels of all three p-tau biomarkers increase with the severity of Alzheimer’s disease, indicating their utility in tracking disease progression and monitoring treatment outcomes.

A recent study suggests that certain plasma tau biomarker assays, particularly those involving p-tau181 and p-tau217, have high levels of accuracy that are comparable to cerebrospinal fluid and PET biomarkers.

Neurofilament light and glial fibrillary acidic protein

The plasma levels of neurofilament light and glial fibrillary acidic protein are also elevated in individuals with Alzheimer’s disease.

Neurofilament light is found in nerve fibers, and higher levels of neurofilament light are an indicator of central nervous system injury, that is, neurodegeneration. Thus, neurofilament light levels are also elevated in other types of dementia , amyotrophic lateral sclerosis , and multiple sclerosis .

Neurofilament light levels do not change much in response to Alzheimer’s severity, limiting its utility as an Alzheimer’s biomarker.

However, neurofilament light could be used to detect the presence of other neurodegenerative disorders in individuals with cognitive impairment who test negative for Alzheimer’s disease-specific biomarkers.

Glial fibrillary acidic protein is a protein that is upregulated in activated astrocytes, a type of brain cell, during the inflammation of the central nervous system, including in Alzheimer’s disease.

Glial fibrillary acidic protein levels are closely associated with amyloid deposit levels in the brain and are correlated with a decline in cognitive function. Moreover, glial fibrillary acidic protein levels are elevated in preclinical Alzheimer’s disease.

Although glial fibrillary acidic protein levels are elevated in other types of dementia, the increase in glial fibrillary acidic protein levels is more profound in Alzheimer’s . Thus, glial fibrillary acidic protein could be used to monitor treatment progress and outcomes in people with Alzheimer’s disease.

Challenges

Although blood-based biomarkers have performed well in studies, there are several challenges to deploying these biomarkers in the clinic. The studies assessing the performance of blood-based biomarkers for Alzheimer’s disease have involved participants selected based on specific criteria, such as the absence of comorbid conditions.

Importantly, these individuals were prescreened for Alzheimer’s disease using brain scans or cerebrospinal fluid biomarkers to categorize them into study groups. The prevalence of Alzheimer’s disease in the population is much lower, and individuals tested in the clinic are expected to show considerable variation in the presentation of diseases than those included in the studies.

Moreover, most of the individuals involved were White, and the performance of these blood-based biomarkers needs to be validated in an ethnically diverse population. These factors have the potential to negatively impact the accuracy of blood-based biomarkers in real-life situations.

There are several ongoing studies to address these concerns about the real-world performance of these assays. For instance, a recent study conducted in primary care and specialized memory care clinics using a kit measuring plasma p-tau217 and the beta-amyloid-42/beta-amyloid-40 ratio showed high diagnostic accuracy exceeding 90%.

There is also a need to develop guidelines for the use of these assays and interpreting their results. There are a number of blood-based biomarker tests that measure plasma p-tau or the beta-amyloid-42/beta-amyloid-40 ratio. These assays use different types of antibodies or different measurement methods, resulting in vastly different outputs.

Thus, there is a need for standardization of these assays and comparison of their performance. Moreover, these tests need to be validated to determine how the changes in biomarker levels correspond to disease severity and established PET or CSF biomarkers.

Schindler told MNT that: “Multiple blood tests for Alzheimer’s disease are now clinically available, but vary widely in their accuracy. No tests are yet FDA-approved.”

“Relatively few healthcare providers have experience using biomarker tests in the diagnosis of Alzheimer’s disease. Given the major impact of an Alzheimer’s disease diagnosis, providers must learn which Alzheimer’s disease blood tests can be trusted, when to order the tests, and how to appropriately interpret the results.”

– Suzanne Schindler, MD, PhD

Lastly, there are pragmatic and logistical challenges to the adoption of these tests in the clinic. Clifford Jack, MD , professor of radiology at the Mayo Clinic, explained that: “Plasma biomarkers are new, and the infrastructure to deliver these tests to the broader medical community is just now starting to roll out. But accessibility will not be a problem in the future.”

Schindler also noted that these blood-based biomarker assays are not covered by insurance, restricting their use.