Blood Tests for Alzheimer’s: What They Can — and Can’t — Tell You

Cognitive Longevity · Biomarkers & Diagnosis
Anton Ostashko, MD·Last medically reviewed July 10, 2026

Overview

Blood biomarkers are changing the evaluation of Alzheimer’s disease, but the phrase “an Alzheimer’s blood test” can be misleading. As of July 10, 2026, the FDA has cleared two plasma tests with different intended uses: the Lumipulse G pTau217/β-amyloid 1-42 ratio for symptomatic adults in specialized care, and Elecsys pTau181 for initial rule-out assessment before specialist referral. Neither is a population-screening test, neither replaces a neurological evaluation, and neither can tell a cognitively healthy person with certainty whether dementia will develop.13

The most important points are:

  • These tests estimate Alzheimer’s-related amyloid biology; they do not directly measure memory, function, or dementia.
  • The exact assay matters. p-tau217, p-tau181, ratios, instruments, cutoffs, and intended populations are not interchangeable.
  • Lumipulse provides negative, intermediate, or positive categories in a specialized-care workup. Elecsys pTau181 is primarily designed to help rule out amyloid pathology before specialty referral.
  • A February 2026 FDA Class II recall affects specified Lumipulse reagent and ratio-related lots. It does not mean that every Lumipulse result is invalid, but affected results may require review.2
  • A result must be interpreted in context. Symptoms, objective cognitive testing, daily function, MRI findings, medical conditions, and the probability of disease before testing all change what a result means.
  • Routine screening of cognitively healthy adults is not established. Amyloid can be present years before symptoms and does not guarantee future dementia.

Evidence cutoff: This article reflects publicly available evidence through July 10, 2026. The main Alzheimer’s Association International Conference is scheduled for July 12–15, 2026 in London and online. Findings scheduled for presentation after this cutoff are not treated here as established results.12

What Does an Alzheimer’s Blood Biomarker Measure?

Alzheimer’s disease is associated with abnormal amyloid-β plaques and tau pathology in the brain. Plasma phosphorylated tau—particularly p-tau217 and p-tau181—can change when Alzheimer’s-related amyloid biology is present. Some tests measure one analyte; others combine two measurements into a ratio to improve classification.

This creates an essential distinction:

  • Biology: Is a pattern associated with Alzheimer’s amyloid pathology likely to be present?
  • Clinical syndrome: Does the person have objective cognitive impairment, how does it affect daily life, and what disease or combination of diseases best explains it?

A biomarker may support a biological classification, but dementia remains a clinical syndrome defined by cognitive and functional impairment. Alzheimer’s pathology may coexist with vascular disease, Lewy body disease, TDP-43 pathology, depression, sleep disorders, medication effects, or other contributors. The same biomarker result can therefore have different implications in different people.5711

The Two FDA-Cleared Plasma Tests Have Different Roles

FeatureLumipulse G pTau217/β-amyloid 1-42 plasma ratioElecsys Phospho-Tau (181P) Plasma
FDA decisionMay 16, 2025October 8, 2025
Intended populationAdults age 50 or older with signs or symptoms of cognitive decline in a specialized-care settingAdults age 55 or older with signs, symptoms, or complaints of cognitive decline during initial assessment before specialist referral
What it measuresRatio of plasma p-tau217 to β-amyloid 1-42Plasma p-tau181 concentration
Reported interpretationNegative, intermediate, or positive for likely amyloid pathologyNegative or positive; a negative result is intended to aid rule-out
Key limitationNot a screening or stand-alone diagnostic test; intermediate or discordant results may need PET or CSF confirmationA positive result has limited rule-in value and requires further evaluation; the test is not recommended once the patient is already referred to a specialist

Lumipulse G pTau217/β-Amyloid 1-42 Plasma Ratio

The FDA-cleared interpretation is assay-specific:

  • Negative: ratio ≤0.00370, consistent with a patient who is unlikely to have amyloid pathology; other causes of cognitive decline should still be evaluated.
  • Intermediate: ratio 0.00371–0.00737, meaning amyloid status is uncertain and further testing should be considered.
  • Positive: ratio ≥0.00738, consistent with likely amyloid pathology; this does not itself diagnose Alzheimer’s disease or another cognitive disorder.

In the 499-person validation study, all participants had a cognitive diagnosis or concern rather than being a healthy screening population. Among positive blood results, 91.8% were amyloid-positive on PET or CSF; among negative blood results, 97.3% were amyloid-negative. Approximately 19.6% of samples fell in the intermediate range.1

Important 2026 recall notice: The FDA posted a Class II recall involving specified Lumipulse ratio instructions and reagent lots after reports that affected material could produce falsely elevated ratios, creating excessive positive or intermediate classifications and reduced specificity compared with amyloid PET or CSF. The recall was initiated December 11, 2025, posted February 5, 2026, and remained listed as open at this article’s evidence cutoff. The cause was listed as under investigation.2

This recall does not mean every Lumipulse result is wrong. A patient whose prior result could affect diagnosis or anti-amyloid treatment should ask the ordering clinician or laboratory whether an affected lot was used and whether the result needs verification. Do not repeat, reinterpret, or act on a result without the clinical team.

Elecsys Phospho-Tau (181P) Plasma

Elecsys pTau181 is intended mainly as an initial rule-out aid. Its FDA-cleared cutoff is 0.722 pg/mL: a result at or below the cutoff is negative and consistent with a negative amyloid PET scan; a result above the cutoff is positive but may not correspond to a positive PET scan and requires additional investigation.3

In a 312-person validation cohort designed to resemble primary care, amyloid-PET prevalence was 13.1%. The assay identified 38 of 41 PET-positive participants (positive percent agreement 92.7%). Its negative predictive value was 97.9%, but its positive predictive value was only 22.4%, because many positive blood results occurred in PET-negative participants in that low-prevalence setting. That does not mean the test is “22% accurate.” It means its useful role in that population is primarily to make amyloid pathology less likely when the result is negative, while a positive result must be worked up further.3

Why Pretest Probability Changes the Meaning of a Result

Predictive value depends on how likely the disease was before testing. In a specialty memory clinic evaluating a patient with objective, progressive amnestic impairment, a positive high-performing biomarker is more likely to represent clinically relevant Alzheimer’s pathology. In a generally healthy person with no symptoms, the same test can generate more false-positive or difficult-to-interpret results because the starting probability is lower.

This is why a test cannot be judged only by a headline sensitivity or specificity. Age, symptom pattern, cognitive testing, functional change, MRI findings, kidney function, anemia, body composition, assay platform, sample handling, and the selected cutoff may all affect interpretation. A 2026 study specifically found that kidney dysfunction, body mass index, and anemia can shift p-tau217 values and influence classification strategies.8

How the 2025 Alzheimer’s Association Guideline Uses Blood Biomarkers

The guideline applies to people with objective cognitive impairment who are undergoing a comprehensive evaluation in specialized care. It does not endorse every available commercial assay, and it does not treat all p-tau tests as equivalent.4

  • A test used for triage should have at least 90% sensitivity and 75% specificity against an accepted amyloid reference standard. A negative result may help rule out amyloid pathology; a positive result should generally be confirmed with amyloid PET or CSF.
  • A test used as a confirmatory substitute for PET or CSF should meet the more demanding threshold of at least 90% sensitivity and 90% specificity in an appropriately validated population.
  • Performance should be demonstrated for the exact commercial assay, platform, specimen handling, and cutoffs used in practice.

These thresholds are not a do-it-yourself checklist for interpreting laboratory results. They are standards for clinicians and health systems deciding whether a particular test is suitable for a particular role.

Why Routine Screening of Cognitively Healthy Adults Is Not Established

Amyloid can accumulate long before symptoms. In a large meta-analysis, amyloid positivity among cognitively normal adults increased substantially with age and APOE ε4 status, suggesting a long interval—often estimated at 20 to 30 years—between initial amyloid positivity and dementia in those who eventually progress.6

A positive result in an asymptomatic person therefore does not answer the questions most people care about: Will I develop cognitive impairment? When? How severe will it be? Will another disease matter more? Current anti-amyloid therapies are labeled for mild cognitive impairment or mild dementia due to Alzheimer’s disease, not for cognitively unimpaired people.910

Screening can also create anxiety, affect family planning and insurability considerations, and start a cascade of PET scans, lumbar punctures, or repeat testing without a proven treatment pathway. Biomarker disclosure in research is performed with structured counseling and safeguards for these reasons.

What a Complete Cognitive Evaluation Still Requires

A blood test should come after the clinical question is defined, not before. A comprehensive evaluation may include:

  • A detailed history of cognitive, behavioral, and functional change, ideally with information from a knowledgeable care partner
  • Neurological examination and standardized cognitive testing
  • Review of medications, alcohol or substance exposure, sleep, mood, hearing, vision, and vascular risk
  • Laboratory assessment for potentially reversible or contributing conditions when appropriate
  • Brain MRI, or CT when MRI is not feasible, to evaluate vascular injury, tumors, hydrocephalus, hemorrhage, and patterns of atrophy
  • Formal neuropsychological testing when the pattern or severity needs more detailed characterization
  • Amyloid PET, CSF, or a validated blood biomarker when identifying Alzheimer’s biology will change diagnosis, prognosis, treatment, or trial eligibility

This staged approach is consistent with current diagnostic-evaluation guidance: identify the syndrome and its functional stage, consider competing or contributing causes, and then use disease biomarkers selectively.7

When PET or CSF May Still Be Needed

  • The blood result is intermediate or analytically invalid.
  • The result conflicts with the history, cognitive profile, MRI, or another biomarker.
  • The assay has not been independently validated for the intended clinical population.
  • A high-stakes treatment decision requires greater certainty.
  • There is concern that a medical condition or preanalytical problem affected the result.
  • The clinician needs information that the blood assay does not provide, such as a different biomarker profile.

No single pathway fits every patient. Availability, insurance coverage, contraindications, patient preference, and the clinical question all matter.

Questions to Ask Before Ordering a Test

  1. What exact test and platform is being ordered? “p-tau” is not specific enough.
  2. Was this assay validated in people like me? Age, symptoms, care setting, and prevalence matter.
  3. What decision will the result change? A test without a defined next step may create information without benefit.
  4. What will a negative, intermediate, or positive result lead to? Ask whether PET or CSF confirmation may still be needed.
  5. Could kidney disease, anemia, obesity, sample handling, or another medical factor affect interpretation?
  6. Was an affected Lumipulse lot involved? This is relevant only to the specific 2025–2026 recall event.
  7. What are the costs and coverage? Coverage varies by assay, laboratory, indication, Medicare contractor, and insurer.

The Bottom Line

Alzheimer’s blood biomarkers are an important advance because they can make assessment of amyloid pathology more accessible. Their value depends on precision: the right assay, the right patient, the right care setting, and a clearly defined clinical question. A positive result is not a diagnosis of dementia; a negative result does not explain every cognitive symptom; and an intermediate or discordant result may require PET or CSF.

At Los Altos Neurology, blood biomarkers are interpreted as part of a staged cognitive evaluation. The diagnosis begins with symptoms, daily function, examination, cognitive testing, medical review, and structural imaging when appropriate. Biomarkers are added when they can clarify cause or guide a treatment decision, and results are discussed with their assay-specific limitations rather than presented as a stand-alone answer.

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References

  1. U.S. Food and Drug Administration. Lumipulse G pTau217/β-Amyloid 1-42 Plasma Ratio: 510(k) Decision Summary (K242706). May 16, 2025.
  2. U.S. Food and Drug Administration. Medical Device Recall Event 98232: Lumipulse G pTau217/β-Amyloid 1-42 Plasma Ratio and related components. Recall initiated December 11, 2025; posted February 5, 2026.
  3. U.S. Food and Drug Administration. Elecsys Phospho-Tau (181P) Plasma: 510(k) Decision Summary (K252163). October 8, 2025.
  4. Palmqvist S, Whitson HE, Allen LA, et al. Alzheimer’s Association clinical practice guideline on the use of blood-based biomarkers in the diagnostic workup of suspected Alzheimer’s disease within specialized care settings. Alzheimer’s & Dementia. 2025;21:e70535. doi:10.1002/alz.70535.
  5. Jack CR Jr, Andrews JS, Beach TG, et al. Revised criteria for diagnosis and staging of Alzheimer’s disease: Alzheimer’s Association Workgroup. Alzheimer’s & Dementia. 2024;20:5143–5169. doi:10.1002/alz.13859.
  6. Jansen WJ, Ossenkoppele R, Knol DL, et al. Prevalence of Cerebral Amyloid Pathology in Persons Without Dementia: A Meta-analysis. JAMA. 2015;313(19):1924–1938. doi:10.1001/jama.2015.4668.
  7. Alzheimer’s Association DETeCD-ADRD Workgroup. Clinical practice guideline for the diagnostic evaluation, testing, counseling, and disclosure of suspected Alzheimer’s disease and related disorders. Alzheimer’s & Dementia. 2024. doi:10.1002/alz.14337.
  8. Yun J, et al. Plasma Phosphorylated Tau 217 Cutoffs for Amyloid Pathology and Kidney Function, Body Mass Index, and Anemia. JAMA Neurology. Published online February 2, 2026. doi:10.1001/jamaneurol.2025.5530.
  9. U.S. Food and Drug Administration. LEQEMBI (lecanemab-irmb) Prescribing Information. 2025.
  10. U.S. Food and Drug Administration. KISUNLA (donanemab-azbt) Prescribing Information. 2025.
  11. Alzheimer’s Association. Criteria for Diagnosis and Staging of Alzheimer’s Disease. Updated 2024.
  12. Alzheimer’s Association. Alzheimer’s Association International Conference 2026. July 12–15, 2026, London and online.

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