Author: Pari Dhayagude
Editor: Grace Stroman
Alzheimer’s disease (AD) remains one of the most challenging neurodegenerative disorders to diagnose in its early stages.1 Before the 2000s, a postmortem autopsy was the only definitive method for diagnosing AD. Since then, progress has been made with magnetic resonance imaging (MRI), positron emission tomography (PET), and cerebrospinal fluid (CSF) assays, but all are costly and invasive, which limits access for many patients.2,3 Cognitive tests, such as the Alzheimer’s Disease Assessment Scale – Cognitive Test (ADAS-Cog), help monitor cognitive symptoms in patients already diagnosed with AD but do not reliably detect changes during mild cognitive impairment (MCI), the precursor to AD.4,5 However, in 2025, the FDA approved the Lumipulse G pTau217/β-Amyloid 1-42 Plasma Ratio test developed by Fujirebio Diagnostics, as the first blood-based diagnostic aid for patients with cognitive impairment undergoing AD assessment.6 Using a simple blood draw, this test measures the ratio of two key proteins, phosphorylated tau 217 (pTau217) and β-amyloid 1-42 (Aβ1-42), providing a less invasive, more accessible, and affordable method for early detection and diagnosis.7 Although the test is currently approved to support clinical evaluation rather than serve as a standalone diagnosis, it represents a significant step forward in early AD diagnosis.
Overview of Alzheimer’s Disease
Alzheimer’s disease is the leading cause of dementia, accounting for approximately 60-80% of cases.8 Driven by the aging U.S. population, the number of new AD cases per year is expected to double by 2060.9 AD is characterized by progressive cognitive decline that affects memory, reasoning, language, and daily activity.10 The disease progresses through stages, from preclinical AD to MCI and eventually to moderate and severe dementia over several years.11 Individuals with AD often experience comorbidities such as depression, anxiety, cardiovascular disease, and diabetes, complicating both diagnosis and disease management.12,13 Because of the nature of disease progression and the presence of symptoms, pathological changes in the brain begin almost a decade before noticeable cognitive decline.14
AD Pathology
AD pathology is characterized by two abnormal protein aggregates: extracellular amyloid-beta (Aβ) plaques and intracellular neurofibrillary tangles composed of hyperphosphorylated tau protein.15 The amyloid cascade hypothesis suggests that the accumulation of Aβ peptides triggers downstream tau hyperphosphorylation at multiple sites, leading to the formation of helical filaments and neurofibrillary tangles throughout the brain.16 These pathological changes result in synaptic loss, neuroinflammation, and progressive neurodegeneration, primarily affecting the hippocampus and entorhinal cortex before spreading to cortical regions.17
Current Diagnostic Methods
Clinical Assessment
AD is first evaluated through a comprehensive assessment that includes patient history, a physical examination, and a battery of neuropsychological tests, including the Mini-Mental State Examination (MMSE) and Montreal Cognitive Assessment (MoCA).18,19 While clinical assessments help quantify cognitive deficits and track disease progression, they are limited in their ability to distinguish AD from other types of dementia.
Neuroimaging
Neuroimaging modalities such as MRI and PET improve AD diagnosis by allowing for visualization of pathologies in living patients, something that was not possible a few decades ago. Structural MRIs can characterize patterns of brain atrophy, particularly in the medial temporal lobe, and PET imaging using different radiotracers can detect both Aβ plaque deposition and neurofibrillary tangle distribution.20,21,22 However, these methods are costly, require specialized facilities, and, for PET, expose patients to radiation, limiting widespread accessibility.23
CSF Analysis
CSF biomarkers have high accuracy in detecting AD pathology, with core biomarkers including decreased Aβ1-42, increased total tau, and increased phosphorylated tau.24 While these tests have a high sensitivity and specificity, the lumbar puncture itself is an invasive procedure with several risks and side effects, including infection, bleeding, pain, and adverse reaction to anesthesia, with additional challenges in elderly patients with spinal abnormalities or those taking anticoagulants.25
How The Lumipulse G pTau217/β-Amyloid 1-42 Plasma Ratio Works
The Lumipulse G pTau217/β-Amyloid 1-42 is a blood-based diagnostic test developed by Fujirebio Diagnostics that uses chemiluminescent enzyme immunoassay (CLEIA) technology to measure two key biomarkers: phosphorylated tau at threonine 217 (pTau217), which reflects tau pathology, and β-amyloid 1-42 (Aβ1-42), which inversely correlates with amyloid plaque burden in the brain.26,27 The monoclonal antibodies can detect these proteins at low concentrations present in plasma, with results available within 30 minutes per sample.26 This diagnostic test is advantageous because it captures changes in both Aβ and tau pathologies simultaneously and presents them in a ratio that is easy to interpret. In individuals with AD, pTau217 levels are elevated due to active tau phosphorylation, while Aβ1-42 levels are reduced because the peptide is sequestered into brain plaques.28 A positive value suggests the presence of AD pathology, while a negative value suggests the absence of AD pathology.29 It is important to note that this test has only been FDA-approved for diagnosing AD in conjunction with a clinical evaluation.
Clinical Validation
The Lumipulse G pTau217/β-Amyloid 1-42 Plasma Ratio test demonstrated robust performance in clinical validation studies. The study submitted to the FDA contained data from a multi-center clinical study of 499 individual plasma samples from cognitively impaired adults to assess the diagnostic performance of the Lumipulse G pTau217/β-Amyloid 1-42 Plasma Ratio test.6 Plasma samples were tested using the Lumipulse G assay and compared with either amyloid PET imaging or an FDA-cleared CSF biomarker test as reference standards. The test demonstrated a positive predictive value (PPV) of 91.7%, meaning that 91.7% of individuals with positive Lumipulse results had confirmed amyloid pathology by PET or CSF analysis.6 The negative predictive value (NPV) was 97.3%, meaning that the vast majority of individuals with negative results were confirmed to lack amyloid pathology.6 Less than 20% of the samples had an indeterminate result.
Additional large-scale studies have validated this assay, including validation of both pTau217 and Aβ1-42 separately. pTau217 is one of the most accurate plasma biomarkers for detecting AD pathology. Palmqvist et al. (2020) evaluated plasma pTau217 across three independent cohorts totaling over 1,500 participants, finding that it distinguished AD from other neurodegenerative disorders with similar accuracy to PET and CSF-based measures, and it performed significantly better than other plasma and MRI-based biomarkers.30 Another study found that pTau217 levels start to rise about 20 years before expected symptom onset in autosomal-dominant AD and correlate strongly with CSF pTau217 and PET signal, further establishing pTau217 as a marker of AD-related tau pathology.31 Other studies have validated that decreased plasma Aβ reflects amyloid sequestration into brain plaques.32 Because pTau217 increases as Aβ1-42 decreases in AD, the ratio helps show greater separation between the two, improving accuracy and reducing indeterminate results compared to single analyte approaches.29
Limitations and Concluding Remarks
While the Lumipulse G pTau217/β-Amyloid 1-42 Plasma Ratio test has significant advantages, it is also important to acknowledge limitations. It is FDA-approved, but only for an aid to diagnosis rather than a standalone diagnostic tool.6 Additionally, about 20% of results fall into an indeterminate zone requiring confirmatory PET or CSF testing, and tests may be affected by comorbidities such as chronic kidney disease.6,33 This test also cannot reliably distinguish AD from all taupathies or identify mixed pathologies, often common in elderly patients, and it requires the LUMIPULSE G platform, which may not be available in all clinical settings.34 Nonetheless, the FDA approval of this test offers a minimally invasive, cost-friendly, accessible pathway to detect AD pathology that was previously only possible through PET imaging or a lumbar puncture.
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