Advances in biomarkers for Alzheimer’s disease and vascular cognitive impairment and dementia: current status and clinical implications

Abstract

Alzheimer’s disease (AD) and vascular cognitive impairment and dementia (VCID) are the leading causes of dementia, and are characterized by distinct yet overlapping pathophysiological mechanisms. AD is characterized by amyloid-beta plaques, neurofibrillary tangles, and neurodegeneration, whereas VCID arises from vascular damage, including small vessel disease and blood-brain barrier disruption. Biomarkers have revolutionized dementia research and offer tools for early diagnosis, monitoring, and therapeutic evaluation. Cerebrospinal fluid (CSF) biomarkers, such as amyloid-beta (Aβ) 42/Aβ40 ratio and phosphorylated tau, provide reliable indicators of AD pathology, while emerging markers like microtubule binding region (MTBR)-tau243 offer insights into disease progression. Blood-based biomarkers, including plasma Aβ42/Aβ40 ratio, phosphorylated tau, neurofilament light chain (NfL), and glial fibrillary acidic protein, represent scalable, non-invasive alternatives. In VCID, biomarkers like matrix metalloproteinases, soluble platelet-derived growth factor receptor-β and inflammatory markers reflect vascular pathology and neurodegeneration. Advances in detection technologies such as single-molecule arrays have improved sensitivity and precision, facilitating their integration into clinical practice. However, challenges remain, including assay variability, limited accessibility, and high costs. Harmonization of protocols and integration of multimodal biomarkers, including CSF, blood, and imaging data, offer a holistic approach to diagnostics. Biomarkers are central to personalized medicine, enabling tailored interventions and improving the outcomes of patients with dementia. Ongoing innovation holds promise for advancing the understanding and management of these complex disorders.

 Introduction

Alzheimer’s disease (AD) is a chronic, progressive neurodegenerative disorder that affects more than 50 million people globally and is a leading cause of dementia. This number is projected to exceed 150 million by 2050¹⁾. Pathological hallmarks of AD include extracellular amyloid-beta (Aβ) plaques, intracellular neurofibrillary tangles of hyperphosphorylated tau, and widespread neurodegeneration²⁾ (Fig. 1). Traditionally, these features have been used for definitive postmortem diagnoses. However, the advent of biomarkers has revolutionized the ability to detect and quantify these pathologies in vivo, providing critical insights for early diagnosis, disease progression monitoring, and therapeutic interventions.

Fig. 1  The pathophysiological mechanisms underlying AD

Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by the abnormal accumulation of amyloid-β (Aβ) and tau proteins. The overproduction and impaired clearance of Aβ42 result in its aggregation into amyloid plaques, disrupting the neuronal microenvironment. Concurrently, Aβ42 levels in the bloodstream and cerebrospinal fluid (CSF) decline, serving as a key biomarker for AD progression.

Pathological tau undergoes hyperphosphorylation, forming neurofibrillary tangles that impair intracellular transport and contribute to neuronal dysfunction and death. This process releases phosphorylated and total tau into the CSF, further indicating neuronal damage. The interplay between Aβ42 aggregation and tau pathology drives neurodegeneration, highlighting their roles as critical biomarkers in AD.

Abbreviations: Aβ, amyloid-β; p-tau, phosphorylated tau; t-tau, total tau.

Vascular cognitive impairment and dementia (VCID) is the second most common cause of dementia and is characterized by cognitive deficits due to vascular damage³⁾. Despite the significant overlap in clinical presentation with AD, VCID stems from different pathophysiological processes such as cerebral small vessel disease, hypoperfusion, and blood-brain barrier (BBB) disruption³⁾. The identification and application of specific biomarkers for VCID remain in their infancy, yet they hold great potential for enhancing diagnostic precision and guiding treatment strategies.

 Biomarkers in Alzheimer’s disease

 Cerebrospinal fluid (CSF) biomarkers

CSF biomarkers offer direct access to the central nervous system (CNS) environment, making them highly reliable indicators of AD pathology. They are integral to the amyloid/tau/neurodegeneration (ATN) classification system, which reframes AD as a biological construct rather than a clinical syndrome. Recent updates to the ATN framework emphasize the importance of incorporating multimodal biomarkers, including advanced neuroimaging and fluid-based metrics, to enhance diagnostic precision and capture the heterogeneity of AD pathology⁴⁾.

 Aβ42 and Aβ42/Aβ40 ratio

Aβ42 is a major component of amyloid plaques. In AD, CSF levels of Aβ42 are significantly reduced due to its aggregation and deposition in the brain⁵⁾. However, Aβ42 levels alone can vary due to differences in total Aβ production among individuals, potentially limiting diagnostic accuracy. Therefore, the Aβ42/Aβ40 ratio has emerged as a reliable biomarker⁶⁾. Aβ40 remains relatively stable in the CSF and serves as a normalization factor for Aβ42 levels, enhancing diagnostic sensitivity and specificity.

Studies have demonstrated that changes in the Aβ42/Aβ40 ratio occur early in the disease continuum, often preceding clinical symptoms. For example, the Alzheimer’s Disease Neuroimaging Initiative (ADNI) study reported that a cutoff value of 192 pg/mL for Aβ42 and 0.39 for the Aβ42/tau ratio predicted AD development with 70% accuracy over 3 years⁷⁾. These findings underscore the utility of this biomarker in identifying and distinguishing preclinical AD from other neurodegenerative disorders.

 Phosphorylated tau (p-tau) and total tau (t-tau)

P-tau biomarkers, specifically p-tau181 and p-tau217, have demonstrated potential in diagnosing AD and predicting cognitive decline. P-tau217 outperforms p-tau181 in distinguishing AD pathology, correlating more strongly with Aβ and tau positron emission tomography (PET) imaging. Both biomarkers are linked to the risk of mild cognitive impairment in amyloid-positive individuals, although differences in their prognostic value for cognitive outcomes are subtle⁸⁾.

Total tau in the cerebrospinal fluid reflects the extent of neuronal damage and neurodegeneration⁹⁾. In the CSF of patients with Alzheimer’s disease, phosphorylated and total tau levels are increased¹⁰⁾.

 Emerging CSF biomarkers

 Microtubule binding region (MTBR)-tau243:

A novel biomarker specific to insoluble aggregated tau, MTBR-tau243, strongly correlated with tau PET imaging and cognitive decline, particularly in advanced disease stages. Additionally, CSF biomarkers allow for the tracking of disease progression, with MTBR-tau243 showing significant changes in advanced stages compared to p-tau, which plateaus earlier. This dynamic behavior makes MTBR-tau243 a valuable tool for monitoring late-stage pathologies¹¹⁾.

 Blood-based biomarkers

Blood-based biomarkers have gained considerable attention owing to their non-invasive nature and scalability, making them suitable for large-scale screening and longitudinal monitoring.

 Plasma Aβ42/Aβ40 ratio

Plasma measurements of the Aβ42/Aβ40 ratio correlate with amyloid PET imaging, providing a blood-based alternative for the detection of cerebral amyloid pathologies. Aβ40/Aβ42 ratios and amyloid-β precursor protein 669–711/Aβ42, as well as their composites, strongly correlate with PiB PET imaging in predicting brain Aβ burden. These findings underscore the potential of composite plasma biomarkers to achieve high diagnostic accuracy for amyloid status, enabling minimally invasive and cost-effective alternatives to traditional methods¹²⁾. Although promising, the small fold-change between AD and non-AD populations poses challenges for clinical implementation¹³⁾. Standardization of assays is crucial for their widespread adoption.

 P-tau

Blood p-tau biomarkers, including p-tau181, p-tau217^14,15), and p-tau231¹⁶⁾, have revolutionized AD diagnostics. These biomarkers demonstrated strong correlations with tau PET imaging and exhibited marked amyloid-dependent changes across the AD continuum. Longitudinal studies have shown that increased plasma p-tau217 levels are associated with clinical deterioration and brain atrophy in patients with preclinical AD¹⁵⁾.

 Neurofilament light chain (NfL) and glial fibrillary acidic protein (GFAP)

Plasma NfL reflects axonal damage, while GFAP is associated with astrocytic activation. Both biomarkers are gaining recognition for their roles in monitoring disease progression and differentiating between neurodegenerative conditions¹⁷⁾.

 Technological advances

Breakthroughs in detection methods such as single-molecule arrays and immunoprecipitation-mass spectrometry have improved the sensitivity and reliability of blood biomarker assays, facilitating their integration into clinical practice¹⁸⁾.

 Cerebral amyloid angiopathy (CAA) biomarker

CAA frequently coexists with AD, with studies reporting moderate-to-severe CAA in approximately 50% of AD patients¹⁹⁾. Furthermore, the Religious Orders Study revealed a prevalence of CAA pathology in 84.9% of autopsied brains²⁰⁾, highlighting the significant overlap between these two conditions. This co-occurrence underscores the importance of distinguishing biomarkers for diagnosing and managing CAA in the context of AD. Recent studies have several promising biomarkers for CAA in the context of Alzheimer’s disease. Clinical studies demonstrate that reduced levels of Aβ40 in CSF and plasma correlate with the progression of CAA, offering specificity in distinguishing it from AD alone²¹⁾. Furthermore, post-mortem analyses reveal significantly greater Aβ40 deposition in CAA-affected brain regions compared to those primarily affected by AD²²⁾. Other candidates including NfL, matrix metalloproteinases (MMPs), complement 3 and lactadherin have also been reported to be associated with CAA based on CSF levels but require further validation through larger, longitudinal studies²¹⁾.

 Future directions in AD biomarker research

Biomarkers play a pivotal role in the early detection, monitoring, and management of AD. Biomarkers such as Aβ42/Aβ40 and p-tau217 facilitate the detection of preclinical AD, enabling early therapeutic intervention²³⁾. Identifying pathological changes before the manifestation of clinical symptoms is critical for initiating timely and potentially disease-modifying treatments. Emerging biomarkers, such as MTBR-tau243, provide valuable insights into treatment efficacy and disease progression, particularly in therapies targeting tau pathology¹¹⁾. These biomarkers allow for the precise evaluation of the therapeutic impact and help tailor strategies for individual patients. They enable the effective identification of at-risk individuals and facilitate targeted enrollment in research aimed at disease prevention or early intervention.

However, several significant challenges remain unresolved. Variability in assay techniques and pre-analytical conditions remains a significant obstacle affecting the reproducibility of results across laboratories. Harmonization of protocols and methodologies is essential to ensure consistency and reliability. Additionally, the high costs of biomarker assays and imaging modalities limit their widespread accessibility, particularly in resource-limited settings. Strategies for reducing costs and enhancing affordability are crucial for equitable healthcare delivery.

Integrating CSF, blood, and imaging biomarkers has the potential to improve diagnostic accuracy by leveraging their complementary strengths. Multimodal approaches can redefine the standards for AD diagnosis and prognosis, offering a more holistic view of disease progression.

Biomarkers are at the forefront of personalized medicine and enable tailored therapeutic strategies that optimize treatment outcomes while minimizing risks. Personalized approaches have been proposed to revolutionize AD care by targeting individual pathophysiological profiles. Efforts to reduce the cost of biomarker assays and expand their use in diverse populations are vital to ensure equitable access to these advancements.

CSF and blood biomarkers have transformed the landscape of AD diagnosis and prognosis. While CSF biomarkers remain the gold standard, blood-based biomarkers are rapidly emerging as indispensable tools because of their non-invasive and scalable nature. Advances in technology combined with the integration of biomarkers into frameworks such as ATN have paved the way for early and precise diagnosis, facilitating the development of effective therapeutic strategies in the era of precision medicine. Continued innovation in biomarker research, coupled with efforts to improve accessibility and standardization, will further solidify their role in AD care. The future of biomarker research holds great promise, offering an enhanced understanding of AD, enabling earlier interventions, and ultimately improving patient outcomes worldwide.

 Biomarkers in vascular cognitive impairment and dementia

VCID primarily arises from cerebrovascular disease (CVD), which includes macro- and microinfarctions, ischemic lesions, and intracranial hemorrhages³⁾. While large vessel disease (LVD) and small vessel disease (SVD) are significant contributors, SVD is particularly prominent and encompasses changes in the small arteries, arterioles, venules, and capillaries. Pathological features, such as lacunes, white matter lesions, microbleeds, and enlarged perivascular spaces, often result from ischemia or chronic hypoperfusion, which disrupts the BBB, induces oxidative stress, and triggers inflammation, ultimately leading to neuronal and glial cell degeneration³⁾ (Fig. 2).

Fig. 2  The pathophysiological mechanisms underlying vascular cognitive impairment and dementia

The pathophysiological mechanisms underlying vascular cognitive impairment and dementia involve various interconnected processes, highlighting potential molecular biomarkers linked to the condition. Endothelial dysfunction reduces nitric oxide production, leading to hypoperfusion and hypoxia, which increase vascular permeability and contribute to the breakdown of the blood-brain barrier (BBB). Damage to the BBB facilitates the infiltration of neutrophils and leukocytes, exacerbating inflammatory responses. Platelet adhesion and coagulation promote microvascular occlusion. Oxidative stress, initiated by fibrinogen and red blood cell extravasation, activates microglia, which generates reactive oxygen species. Pro-inflammatory cytokines such as IL-6, IL-1β, and TNF-α intensify neuronal damage, leading to the release of NfL. Activated astrocytes and microglia further amplify neuroinflammation. Additionally, pericyte degeneration and the activity of matrix metalloproteinases (MMP-2, MMP-3, and MMP-9) contribute to BBB disruption. Biomarkers such as elevated glial fibrillary acidic protein (GFAP) and lipocalin-2 indicate astrocyte activation.

Abbreviations: BBB, blood brain barrier; GFAP, glial fibrillary acidic protein; IL, interleukin; LCN2, lipocalin-2; MMP, matrix metalloproteinase; NfL, neurofilament light chain; NO, nitric oxide; PlGF, placental growth factor, ROS, reactive oxygen species; sPDGFRβ, soluble platelet-derived growth factor receptor-β; TGF-β, transforming growth factor-β; TNF-α, tumour necrosis factor-alpha; VEGF, vascular endothelial growth factor.

Endothelial dysfunction plays a central role in VCID and is exacerbated by aging and vascular risk factors that impair nitric oxide (NO) production through elevated levels of asymmetric dimethylarginine (ADMA), a natural inhibitor of endothelial NO synthase. Reduced NO availability leads to hypoperfusion, hypoxia, and increased vascular permeability, facilitating the extravasation of proinflammatory and neurotoxic molecules into the brain²⁴⁾. Oxidative stress further compounds endothelial dysfunction, with reactive oxygen species amplifying the inflammatory response and impairing vascular autoregulation^25,26). This cascade involves the activation of microglia and astrocytes, which produce inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), interleukin (IL)-6, and IL-1β, directly damaging neurons and oligodendrocytes²⁶⁾.

Breakdown of the BBB is a hallmark of VCID and is driven by the hypoxia-induced upregulation of MMPs that degrade tight junction proteins and the basement membrane²⁷⁾. This allows neurotoxic molecules to infiltrate the brain, contributing to white matter damage and cognitive dysfunction. In addition, the degeneration of neurons and glial cells in SVD is aggravated by oxidative stress and inflammatory cytokines, resulting in myelin loss and axonal damage. Biomarkers such as the NfL and GFAP reflect ongoing neurodegeneration²⁷⁾.

LVD pathological changes in larger arteries involve atherosclerosis and atherothrombosis. Endothelial dysfunction, oxidative stress, and inflammation play key roles in processes such as Low-Density Lipoprotein (LDL) oxidation and foam cell formation, which contribute to vessel wall thickening and lumen narrowing²⁸⁾. Cardioembolic Stroke, often resulting from atrial fibrillation, is another significant cause of VCID, and is characterized by thrombus formation in the heart that occludes the cerebral vessels²⁹⁾. Intracranial hemorrhages, including intracerebral and subarachnoid hemorrhages, further contribute to the pathology of VCID. Intracerebral hemorrhages are often linked to aging, hypertension, and cerebral amyloid angiopathy³⁰⁾, whereas subarachnoid hemorrhages commonly result from aneurysm rupture^31,32).

 CSF biomarkers

CSF biomarkers offer a direct view of central nervous system (CNS) processes and provide valuable insights into endothelial dysfunction, oxidative stress, inflammation, and neurodegeneration in VCID.

Inflammatory markers in the CSF such as IL-6 and vascular endothelial growth factor (VEGF) reflect neurovascular inflammation and angiogenesis. IL-6 levels were higher in patients with VCID than in AD, supporting its specificity for vascular pathology³³⁾. VEGF-A and VEGF-C, subtypes of VEGF, show increased levels in VCID, with VEGF-C being particularly associated with subcortical ischemic vascular dementia (SIVD)³⁴⁾.

MMPs such as MMP-2 and MMP-9, and their inhibitors (tissue inhibitors of metalloproteinases [TIMP]-1 and TIMP-2) have been explored as indicators of BBB breakdown in VCID³⁵⁾. Elevated CSF MMP-9 levels, compared with healthy controls, suggest an active role in vascular endothelial degradation and BBB disruption^36,37).

Soluble Platelet-Derived Growth Factor Receptor-β (sPDGFRβ) is a marker of pericyte injury and early BBB breakdown^38–40). Studies demonstrate that sPDGFRβ increases in CSF during the early stages of cognitive impairment, even before significant amyloid-beta or tau biomarker changes^40–43).

Neurodegenerative markers, such as NfL and GFAP, are increasingly being studied for their role in VCID. CSF NfL levels correlated with white matter hyperintensities (WMH) severity, distinguishing VCID from healthy aging and AD^44–47).

The cerebrospinal fluid/serum albumin quotient, the gold standard for measuring BBB integrity, was significantly higher in patients with VaD than in those with AD. This marker indicates increased BBB permeability, particularly in cases of subcortical small-vessel disease^48,49).

 Blood (serum/plasma) biomarkers

Endothelial Adhesion Molecules, such as intercellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule (VCAM)-1 are crucial for leukocyte migration during ischemic injury. Studies have shown correlations between elevated serum ICAM-1 levels and increased WMH volume⁵⁰⁾, and VCAM-1 has been linked to a higher risk of VCID⁵¹⁾. However, conflicting findings, such as the null results of the Rotterdam Study, indicate variability in their predictive value.

Levels of P-selectin and E-selectin, which mediate leukocyte adhesion, are elevated in patients with severe CVD. These biomarkers reflect endothelial activation, but may not differentiate between specific vascular pathologies^52,53).

ADMA, an endogenous inhibitor of NO synthase and is a reliable marker of vascular dysfunction. Elevated ADMA levels in plasma are associated with impaired vascular autoregulation and higher incidence of WMH and lacunes in VCID⁵⁴⁾. The Arg/ADMA ratio provides additional specificity, as lower ratios are observed in patients with small vessel diseases⁵⁵⁾.

Oxidized Low-Density Lipoprotein (OxLDL), a marker of lipid oxidation, is associated with vascular inflammation and foam cell formation. Elevated plasma OxLDL levels have been observed in patients with VCID⁵⁶⁾; however, some studies have reported inconsistent results. This highlights the need for standardized assays and further research.

Lipoprotein-Associated Phospholipase A2 (Lp-PLA2) binds to oxLDL and has emerged as a robust biomarker. Elevated serum levels of Lp-PLA2 are correlated with severe WMH and silent brain infarcts, emphasizing its role in connecting oxidative stress with vascular injury^50,57).

Inflammatory biomarkers such as IL-6, C-reactive Protein (CRP), and TNF-α have been widely studied. Elevated serum IL-6 and CRP levels are associated with the risk of WMH and VaD risk⁵⁸⁾, though inconsistent findings suggest that these markers are influenced by systemic inflammation. TNF-α levels are higher in multi-infarct dementia but overlap with AD in some studies, underscoring their limited specificity^34,59,60).

Markers of angiogenesis and vascular remodeling, such as VEGF-C, were significantly elevated in the plasma of patients with patients with SIVD and VCID³⁴⁾. These markers are relevant to both systemic and cerebral vascular pathologies.

Fibrinogen, a soluble plasma glycoprotein involved in coagulation, was positively correlated with lacunes and WMH in some prospective studies⁶¹⁾. However, its levels are also elevated in non-CVD conditions such as myocardial infarction, limiting its specificity for VCID.

D-dimer, a product of fibrin degradation, and von Willebrand Factor (vWF), which mediates platelet adhesion, are elevated in VCID and are associated with cognitive decline. High plasma D-dimer levels are linked to Binswanger disease and subcortical VaD, whereas vWF is correlated with WMH severity^62,63).

Thrombomodulin, which is involved in the endothelial regulation and clotting pathways, is a promising marker. Increased plasma plasminogen activator inhibitor-1 levels have been observed in VaD and SIVD, differentiating these conditions from AD⁶⁴⁾.

Neurodegenerative markers in the blood, such as NfL and GFAP, reflect the systemic signs of neuronal and astrocytic damage. Elevated plasma NfL levels correlate with lacunar infarcts⁶⁵⁾, whereas higher GFAP levels are linked to subcortical infarcts and cognitive decline in SVD⁴⁶⁾.

The Placental Growth Factor (PlGF) is critical for endothelial homeostasis. PlGF is a member of the VEGF family that promotes angiogenesis and vascular permeability. Elevated PlGF levels in CSF have been observed in SIVD, correlating with WMH severity and cognitive decline^34,66).

 Future directions in VCID biomarker research

Emerging biomarker panels that integrate multiple markers have shown promise for improving diagnostic specificity and sensitivity. Combining CSF tau, Aβ42, and NfL has been effective in distinguishing subcortical VCID from AD⁶⁷⁾, while panels including inflammatory and vascular biomarkers like MMP-9 and PlGF offer additional diagnostic accuracy³⁴⁾. Advances in machine learning, leveraging data from blood and CSF assays along with neuroimaging, have significant potential for refining biomarker selection and enhancing diagnostic precision⁶⁸⁾.

Although significant progress has been made in identifying VCID biomarkers, challenges remain in terms of specificity, reproducibility, and clinical applicability. Future research should focus on validating biomarker panels, exploring genetic models, such as cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy, and utilizing artificial intelligence to develop robust diagnostic tools⁶⁹⁾. These efforts will pave the way for personalized approaches to diagnose and treat VCID, and ultimately improve patient outcomes.

To comprehensively address dementia, it is essential to investigate the pathology of mixed dementia (MD), which combines AD and VCID. Pathological changes associated with VCID and AD frequently coexist, a condition termed mixed dementia⁷⁰⁾. Autopsy studies report its prevalence ranging from 6% to 12% on average, though some studies indicate a broader range of 2% to 58%⁷¹⁾. Clinically, MD encompasses AD with cerebrovascular disease, vascular lesions, and associated risk factors⁷²⁾. Notably, in the Rush Memory and Aging Project, 40% of dementia cases were identified as mixed dementia⁷³⁾.

Against this backdrop, recent studies suggest that Lipocalin-2 (LCN2) may serve as a potential biomarker for MD due to its association with both neurodegenerative and vascular processes. Studies have shown that LCN2 is significantly elevated in infarct-related brain regions, particularly in reactive astrocytes and macrophages, which indicates its involvement in inflammation and tissue repair processes associated with vascular damage. Moreover, CSF LCN2 levels are higher in mixed dementia compared to AD or healthy controls, suggesting its potential to reflect the combined vascular and neurodegenerative pathologies. The role of LCN2 in neuroinflammation and iron metabolism closely corresponds to the pathological mechanisms underlying both AD and VCID, highlighting its relevance in capturing the complexity of mixed dementia. However, further research is needed to validate its clinical applicability^74–77).

 Conclusion

Biomarkers have transformed the landscape of AD and VCID research, enabling early diagnosis, monitoring disease progression, and evaluation of therapeutic efficacy. While CSF and imaging biomarkers remain the gold standards for AD, blood-based biomarkers are rapidly emerging as practical alternatives. Similarly, research on VCID biomarkers is gaining momentum, with promising candidates that reflect cerebrovascular pathology and neurodegeneration. Continued advancements in biomarker discovery and validation will pave the way for personalized medicine and improve outcomes in patients with dementia.

 Acknowledgements

This paper was supported financially by Katakami Foundation For Clinical Research and the research grant of Astellas Foundation for Research on Metabolic Disorders.

 Disclosures

No conflicts of interest declared.

Table 1 CSF Biomarkers in AD

Biomarker	Key Features	Diagnostic Utility
Aβ42 and Aβ42/Aβ40 Ratio	- Aβ42: Major component of amyloid plaques; reduced in AD due to brain aggregation. - Aβ42/Aβ40 ratio improves diagnostic accuracy by normalizing Aβ42 levels.	- Early detection of AD. - Cutoff values: Aβ42 (192 pg/mL), Aβ42/tau ratio (0.39). - Predicts AD with 70% accuracy over three years.
p-tau	- Reflects tau hyperphosphorylation. - p-tau181, p-tau217. - Strong correlations with Aβ and tau PET imaging.	- P-tau181 and p-tau217 are linked to the risk of mild cognitive impairment in amyloid-positive individuals.
t-tau	- Indicates neuronal damage and neurodegeneration.	- Complements p-tau; less specific to AD but elevated in neurodegenerative conditions.
MTBR-tau243	- Novel marker specific to insoluble aggregated tau. - Strong correlation with tau PET imaging and cognitive decline.	- Tracks late-stage AD progression. - Useful for monitoring advanced disease stages.

Abbreviations: AD, Alzheimer’s disease; Aβ, amyloid-beta; CSF, cerebrospinal fluid; p-tau, phosphorylated tau; t-tau, total tau; MTBR-tau, microtubule binding region tau; PET, positron emission tomography.

Table 2 Blood-Based Biomarkers in AD

Biomarker	Key Features	Diagnostic Utility
Plasma Aβ42/Aβ40 Ratio	- Correlates with amyloid PET imaging. - Provides a blood-based alternative for detecting cerebral amyloid pathology. - Small fold-change between AD and non-AD populations poses challenges.	- Facilitates early detection of AD. - Requires assay standardization for widespread clinical implementation.
p-tau	- Includes variants such as p-tau181, p-tau217, and p-tau231. - Strong correlations with tau PET imaging. - Longitudinal increases in p-tau217 associated with clinical deterioration and brain atrophy.	- Revolutionized AD diagnostics. - Effective in detecting amyloid-dependent changes across the AD continuum.
NfL and GFAP	- NfL: Reflects axonal damage. - GFAP: Associated with astrocytic activation.	- Monitors disease progression. - Differentiates between neurodegenerative conditions.

Abbreviations: Aβ, amyloid-beta; AD, Alzheimer’s disease; GFAP, glial fibrillary acidic protein; NfL, neurofilament light chain; PET, positron emission tomography; p-tau, phosphorylated tau.

Table 3 CSF Biomarkers in VCID

Biomarker	Key Features	Diagnostic Utility
Inflammatory Markers (IL-6, VEGF)	- IL-6: Reflects neurovascular inflammation; higher in VCID compared to AD. - VEGF-A and VEGF-C: VEGF-C is associated with SIVD.	- Differentiates VCID from AD. - Highlights neurovascular inflammation and angiogenesis.
MMP-2, MMP-9 and TIMPs	- Indicators of BBB breakdown. - Elevated MMP-9 levels suggest vascular endothelial degradation and BBB disruption.	- Markers of BBB integrity and endothelial damage.
sPDGFRβ	- Marker of pericyte injury and early BBB breakdown. - Elevated in early cognitive impairment before significant amyloid-beta or tau changes.	- Early marker of BBB dysfunction and vascular cognitive impairment.
Neurodegeneration Markers (NfL, GFAP)	- NfL: Correlates with WMH severity; distinguishes VCID from healthy aging and AD. - GFAP: Reflects astrocytic remodeling.	- Tracks neurodegeneration and differentiates VCID from other conditions.
CSF/Serum Albumin Quotient	- Gold standard for measuring BBB integrity. - Higher in VCID compared to AD; highlights increased BBB permeability in subcortical small vessel disease.	- Indicates BBB dysfunction and increased permeability, especially in subcortical small vessel disease.

Abbreviations: AD, Alzheimer’s disease; BBB, blood-brain barrier; CSF, cerebrospinal fluid; GFAP, glial fibrillary acidic protein; IL-6, interleukin-6; MMP, matrix metalloproteinase; NfL, neurofilament light chain; sPDGFRβ, soluble platelet-derived growth factor receptor-β; SIVD, subcortical ischemic vascular disease; TIMP, tissue inhibitor of metalloproteinases; VCID, vascular cognitive impairment and dementia; VEGF, vascular endothelial growth factor; WMH, white matter hyperintensities.

Table 4 Blood Biomarkers in VCID

Biomarker	Key Features	Diagnostic Utility
Endothelial Adhesion Molecules (ICAM-1, VCAM-1)	- Crucial for leukocyte migration during ischemic injury. - ICAM-1: Correlates with WMH volume. - VCAM-1: Linked to higher VCID risk.	- Reflects endothelial dysfunction. - Predictive value varies across studies.
P-selectin and E-selectin	- Mediate leukocyte adhesion. - Elevated in severe CVD cases.	- Indicates endothelial activation. - Limited differentiation between specific vascular pathologies.
ADMA	- Endogenous inhibitor of nitric oxide synthase. - Elevated ADMA: Associated with impaired vascular autoregulation, WMH, and lacunes. - Arg/ADMA ratio provides additional specificity.	- Identifies small vessel disease. - Highlights vascular dysfunction in VCID.
OxLDL and Lp-PLA2	- OxLDL: Marker of lipid oxidation linked to vascular inflammation. - Lp-PLA2: Correlates with severe WMH and silent brain infarcts.	- Reflects oxidative stress and vascular injury. - Highlights need for standardized assays.
Inflammatory Biomarkers (IL-6, CRP, TNF-α)	- IL-6 and CRP: Associated with WMH and VaD risk. - TNF-α: Higher in multi-infarct dementia but overlaps with AD.	- Reflects systemic inflammation. - Limited specificity for VCID.
Markers of Angiogenesis (VEGF-C, PlGF)	- VEGF-C: Elevated in SIVD and VCID patients. - PlGF: Supports angiogenesis and vascular permeability; correlates with WMH severity.	- Relevant for systemic and cerebral vascular pathologies. - Tracks vascular remodeling.
Fibrinogen and D-dimer	- Fibrinogen: Correlates with lacunes and WMH but elevated in non-CVD conditions. - D-dimer: Linked to Binswanger’s disease and subcortical VCID.	- Indicates coagulation pathway involvement. - Limited specificity due to overlap with other conditions.
vWF	- Mediates platelet adhesion. - Correlates with WMH severity.	- Tracks vascular injury severity.
TM and PAI-1	- TM: Involved in endothelial regulation. - PAI-1: Elevated in VaD and SIVD, differentiating these conditions from AD.	- Reflects endothelial regulation and clotting pathway involvement.
Neurodegeneration Markers (NfL, GFAP)	- NfL: Correlates with lacunar infarcts. - GFAP: Linked to subcortical infarcts and cognitive decline in SVD.	- Tracks neuronal and astrocytic damage in VCID.
PlGF	- Supports angiogenesis and vascular permeability. - Correlates with WMH severity and cognitive decline in SIVD.	- Indicates endothelial dysfunction and vascular injury.

Abbreviations: AD, Alzheimer’s disease; ADMA, asymmetric dimethylarginine; Arg, arginine; BBB, blood-brain barrier; CVD, cerebrovascular disease; CRP, C-reactive protein; CSF, cerebrospinal fluid; GFAP, glial fibrillary acidic protein; ICAM-1, intercellular adhesion molecule-1; IL-6, interleukin-6; Lp-PLA2, lipoprotein-associated phospholipase A2; NfL, neurofilament light chain; OxLDL, oxidized low-density lipoprotein; PAI-1, plasminogen activator inhibitor-1; PET, positron emission tomography; PlGF, placental growth factor; sPDGFRβ, soluble platelet-derived growth factor receptor-β; SIVD, subcortical ischemic vascular disease; TIMP, tissue inhibitor of metalloproteinases; TM, thrombomodulin; TNF-α, tumor necrosis factor-alpha; VCAM-1, vascular cell adhesion molecule-1; VCID, vascular cognitive impairment and dementia; VEGF, vascular endothelial growth factor; vWF, von Willebrand factor; WMH, white matter hyperintensities.

Acknowledgements

This paper was supported financially by Katakami Foundation For Clinical Research and the research grant of Astellas Foundation for Research on Metabolic Disorders.

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