# Simplified Blood Tests Accurately Confirm Alzheimer's Brain Changes Before Starting New Treatments

Two blood tests, when used together, can confirm whether a patient has the brain amyloid buildup that makes them eligible for new Alzheimer's disease-modifying therapies. In a study of nearly 400 Australians, the combination of phosphorylated tau 217 (p-tau217) and the amyloid beta 42/40 (Aβ42/40) ratio achieved over 90% accuracy in detecting amyloid positivity on brain scans. The test performed equally well in a group designed to mimic real-world patients who would seek treatment, with a sensitivity of 99% and specificity of 87%. These results suggest that a simple blood draw could replace more invasive cerebrospinal fluid tests when screening patients for drugs like lecanemab and donanemab.

# Simplified Blood Tests Accurately Confirm Alzheimer's Brain Changes Before Starting New Treatments

## Table of Contents

- Key Points
- Background: Why These Blood Tests Matter
- Study Methods: How the Research Was Conducted
- Study Participants: Two Distinct Groups
- Bridging Results Across Test Batches
- Blood Biomarker Levels Between Amyloid Positive and Negative Groups
- Correlations Between Blood Markers and Brain Scan Amyloid Levels
- Predictive Performance: How Well the Tests Detect Amyloid Positivity
- Clinical Implications: What This Means for Patients
- Limitations: What This Study Couldn't Prove
- Recommendations: What Patients Should Know
- Frequently Asked Questions
- Source Information

## Key Points

- In a study of 397 Australians, combining p-tau217 and Aβ42/40 ratio blood tests detected brain amyloid with over 90% accuracy.
- Among patients like those eligible for new Alzheimer's drugs, sensitivity was 99% and specificity was 87%.
- The blood test met consensus criteria to replace cerebrospinal fluid or PET confirmation for most patients starting lecanemab or donanemab.
- About 10–15% of results fall in an intermediate gray zone, requiring PET or spinal fluid to decide treatment.
- Test results were highly reproducible, but participants were mostly White; broader ethnic applicability remains uncertain.

## Background: Why These Blood Tests Matter

New disease-modifying therapies (DMTs) for Alzheimer's disease (AD) — specifically lecanemab (marketed as Leqembi) and donanemab (marketed as Kisunla) — are now approved in several countries. These drugs work by removing amyloid beta (Aβ) plaques from the brain. To get these treatments, a patient must first show evidence of brain amyloid buildup, called "Aβ positivity."

Currently, confirming Aβ positivity requires either a positron emission tomography (PET) brain scan or a lumbar puncture to collect cerebrospinal fluid (CSF). Both are expensive, time-consuming, and sometimes uncomfortable. Blood-based biomarkers (BBMs) — simple blood tests that measure proteins related to Alzheimer's — have advanced rapidly. However, doctors have lacked clear guidance on which blood test is good enough to confirm Aβ positivity before prescribing a DMT.

In 2023, a consensus statement defined the standards. A blood test can serve as a *confirmatory test* if it matches the performance of CSF biomarkers: at least 90% sensitivity (correctly identifying who has amyloid) and 90% specificity (correctly identifying who doesn't). If the test is not quite that good, it might still work as a *triaging test* — ruling out people who are unlikely to have amyloid — but then those who remain would still need a confirmatory PET scan or CSF test.

The current study aimed to see whether two plasma markers — p-tau217 and the Aβ42/40 ratio, measured with a commercially available Fujirebio Lumipulse test — could meet the confirmatory test standard. The research team also wanted to define specific cut-off values that doctors could use in routine practice.

## Study Methods: How the Research Was Conducted

### Blood sample collection

Blood was drawn from participants between 7:30 and 10:30 a.m. after an overnight fast. Samples were collected into tubes containing a substance to prevent platelet activation, then centrifuged to separate plasma. Plasma was frozen and stored in liquid nitrogen. Samples were later shipped on dry ice to Labcorp-Monogram Biosciences in South San Francisco, USA, for analysis.

Samples were analyzed on a single Fujirebio Lumipulse G1200 instrument. The assays measured p-tau217, Aβ42, and Aβ40. The laboratory validated the tests according to official clinical standards (CAP-CLIA guidelines). Inter-assay precision was excellent — the coefficient of variation (a measure of result variability) was below 20%, and control accuracy was within 20% of expected values. The ADCC cohort was tested using one set of reagent lots (Aβ42 lot #3303, Aβ40 lot #3033, p-tau217 lot #4049). The ITTC cohort was tested about 12 months later using a different lot (Aβ1-42 lot #T6B5081, Aβ1-40 lot #T4B5112, p-tau217 lot #D4C5066).

### PET imaging and amyloid classification

All PET scans were performed within 12 months of the blood draw. Four different PET tracers were used: 11C-Pitttsburgh compound B (PiB), 18F-NAV4694 (NAV), 18F-flutemetamol (FLUTE), or 18F-florbetapir (FBP). Scan data were standardized to the Centiloid (CL) scale, a common measurement unit for brain amyloid.

In the ADCC (Alzheimer's disease continuum cohort), participants were classified as Aβ PET negative if their CL was below 15 and Aβ PET positive if their CL was 25 or higher. This allowed comparisons across the disease continuum. In the ITTC (intention-to-treat cohort), positivity was set at CL ≥25, consistent with the cognitively impaired population.

### Statistical analysis

Researchers used receiver operating characteristic (ROC) curves to calculate area under the curve (AUC), sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy. Cut-offs were derived using two methods: the Youden index (maximizing sensitivity plus specificity) and dual cut-offs (setting both sensitivity and specificity to at least 95%, which leaves a "gray zone" between them).

They also compared models that added demographic factors — age, sex, apolipoprotein E (APOE) ε4 status, and PET tracer type — to the blood biomarkers. A technique called least absolute shrinkage and selection operator (LASSO) was used for multivariate selection of the best combination of markers.

## Study Participants: Two Distinct Groups

The study used samples from the Australian Imaging, Biomarkers, and Lifestyle (AIBL) Study of Ageing. Two separate sub-cohorts were selected, with no participant included in both groups.

### AD continuum cohort (ADCC)

This cohort was designed to assess plasma biomarker levels across the entire Alzheimer's disease spectrum. It included:

- 75 cognitively unimpaired, Aβ PET negative participants (CUAβ–); roughly 50% had subjective memory complaints
- 48 cognitively unimpaired, Aβ PET positive participants (CUAβ+); 66% had subjective memory complaints
- 26 participants with mild cognitive impairment (MCI) who were Aβ PET positive (MCI Aβ+)
- 48 participants with AD dementia who were Aβ PET positive (AD Aβ+)

Total sample: 197 participants. Of these, 62% were Aβ PET positive. Among the cognitively unimpaired group, 39% had preclinical AD (Aβ+), while 61% were Aβ–. There were no significant differences in age or sex between the Aβ groups, but all other clinical parameters differed significantly (p<0.0001). The mean age was 75.1 years (standard deviation 7.3). The mean Centiloid value was 56.7 in the overall sample, −0.5 in Aβ– participants, and 91.9 in Aβ+ participants.

### Intention-to-treat cohort (ITTC)

This cohort was designed to mimic the patient population that would actually present in primary/secondary care and meet the inclusion criteria for a DMT. Eligibility for lecanemab or donanemab in clinical trials required a Clinical Dementia Rating of 0.5–1 and a Mini-Mental State Examination (MMSE) score of 22–28. (Note: approval criteria vary by country — for example, in the US any number of APOE ε4 alleles is allowed, while in Australia and the EU only 0 or 1 copy is allowed.)

The ITTC included:

- 65 MCI Aβ+ participants
- 61 AD dementia Aβ+ participants
- 59 MCI Aβ– participants
- 15 AD dementia Aβ– participants

Total sample: 200 participants. Of these, 62.5% had MCI and 37.5% had mild AD. In the MCI group, 52% were Aβ+; in the AD group, 80% were Aβ+. Overall, 63% were Aβ PET positive. No significant age or sex differences existed between Aβ groups, but other clinical parameters differed significantly (p<0.0001). Mean age was 73.6 years (SD 8.4). Mean Centiloid was 64 overall, −0.6 in Aβ–, and 101.9 in Aβ+.

## Bridging Results Across Test Batches

Because the two cohorts were measured about 12 months apart, the researchers ran a bridging study to make sure results from both batches could be compared fairly. They re-ran 40 samples from the ADCC cohort (10 from each of the four diagnostic groups: CUAβ–, CUAβ+, MCI Aβ+, AD Aβ+) at the same time as the ITTC samples.

Bland–Altman analysis showed small systematic differences between runs. The bias ranged from about a 0.7% decrease for Aβ40 to about a 7% increase for Aβ42 and p-tau217 in the ITTC compared to ADCC. The standard deviation of the differences was small — less than 1 standard deviation of the original ADCC measurements for each biomarker (Aβ40: 0.26 SD, Aβ42: 0.19 SD, p-tau217: 0.12 SD). Concordance correlations for all three assays ranged from 0.93 to 0.99.

Because the variability between the two batches was consistent, all ITTC data were transformed using Deming regression equations so that the results from both cohorts were on the same scale. This ensures that the cut-off values derived from one cohort can be applied to the other.

## Blood Biomarker Levels Between Amyloid Positive and Negative Groups

All four biomarkers — Aβ42, p-tau217, Aβ42/40, and the p-tau217/Aβ42 ratio — were significantly different in Aβ PET+ groups compared to Aβ PET– groups in both cohorts. The differences remained significant even after adjusting for age, sex, APOE ε4 status, and PET tracer (p<0.001 in all cases).

The strongest effect was seen for the p-tau217/Aβ42 ratio. In the ADCC, the mean ratio was 0.006 in Aβ– and 0.026 in Aβ+ (Cohen's D = 2.43, p=1.25E-37 unadjusted; p=1.67E-09 adjusted). In the ITTC, the ratio was 0.006 in Aβ– and 0.030 in Aβ+ (Cohen's D = 2.69, p=2.13E-40 unadjusted; p=1.03E-10 adjusted). Cohen's D is a measure of effect size; values above 0.8 are considered large, and here the values were extremely large.

Individual biomarker means were also reported (values in pg/mL, though the paper doesn't specify units explicitly):

- Aβ42 in ADCC: 27.969 (Aβ–) vs 23.484 (Aβ+), Cohen's D=0.928
- p-tau217 in ADCC: 0.154 (Aβ–) vs 0.596 (Aβ+), D=2.17
- Aβ42/40 in ADCC: 0.092 vs 0.076, D=1.81
- Aβ42 in ITTC: 28.212 (Aβ–) vs 22.853 (Aβ+), D=1.14
- p-tau217 in ITTC: 0.168 vs 0.663, D=2.36
- Aβ42/40 in ITTC: 0.090 vs 0.074, D=2.41

The Aβ42/40 ratio showed significantly larger effect sizes compared to Aβ42 alone in both cohorts (ADCC: p=0.0001; ITTC: p<0.0001). However, the p-tau217/Aβ42 ratio did not significantly outperform p-tau217 alone (ADCC: p=0.318; ITTC: p=0.223).

## Correlations Between Blood Markers and Brain Scan Amyloid Levels

The study also examined how well the blood markers tracked with the actual amount of brain amyloid measured on the Centiloid scale.

Aβ42/40 showed a moderate inverse correlation with CL — meaning lower blood ratios were associated with higher brain amyloid. The correlation coefficient (Rho) was −0.507 in the ADCC and −0.602 in the ITTC. The ratio had less variance than Aβ42 alone, making it a more stable indicator.

Using Aβ42/40 to classify Aβ PET status, only 4% of participants in the ADCC and 2% in the ITTC were false negatives (blood test said negative but scan was positive). False positives (blood positive, scan negative) were 8% in ADCC and 6% in ITTC.

p-tau217 showed strong positive correlations with CL: Rho = 0.732 (ADCC) and 0.723 (ITTC). The p-tau217/Aβ42 ratio correlated even better: Rho = 0.751 (ADCC) and 0.753 (ITTC). Compared to p-tau217 alone, using the p-tau217/Aβ42 ratio reduced false positives from 7% to 4% in the ADCC, with limited change in the ITTC (4% to 5%). False negatives decreased from 4% to 0% in the ITTC.

## Predictive Performance: How Well the Tests Detect Amyloid Positivity

The central question was whether the blood tests could match the performance of CSF biomarkers (at least 90% sensitivity and specificity). The answer was yes — particularly when the two markers were combined.

### ADCC results

In the ADCC, individual biomarker AUCs for predicting Aβ PET status were strongest for p-tau217 and the p-tau217/Aβ42 ratio:

- p-tau217/Aβ42 ratio: AUC = 0.961
- p-tau217 alone: AUC = 0.941

Using the p-tau217/Aβ42 ratio with a cut-off derived from the Youden index, the sensitivity was 93%, specificity 92%, and overall accuracy 93%.

Adding age, sex, and APOE ε4 status to the biomarkers increased the AUC for Aβ42 and the Aβ42/40 ratio only. It did not improve the performance of p-tau217 or the p-tau217/Aβ42 ratio.

In a pre-clinical AD subgroup (cognitively unimpaired participants only), the Aβ42/40 ratio had a higher AUC (0.906) compared to the complete ADCC (0.893), but this difference was not statistically significant (p>0.05). Adding demographic factors did not improve AUC for any biomarker in this subgroup.

### ITTC results

When the same cut-offs derived from the ADCC were applied to the ITTC, the AUC remained virtually identical: 0.959 for the p-tau217/Aβ42 ratio. However, the test characteristics differed in this real-world-like population:

- Sensitivity increased to 99%
- Specificity decreased to 87%
- Overall accuracy increased to 95%

In the ITTC, the composite score (a linear combination of APOE ε4 + age + Aβ42 + p-tau217 + Aβ42/40 + p-tau217/Aβ42), the linear combination of Aβ42/40 with p-tau217 plus age, sex, and APOE ε4, and the p-tau217/Aβ42 ratio alone were all significantly better (nominally, unadjusted) at predicting Aβ PET status compared to p-tau217 alone (p=0.013, p=0.010, and p=0.009, respectively).

### Dual cut-offs and the intermediate zone

The authors also tested a dual cut-off approach, aiming for 95% sensitivity and 95% specificity. This creates an intermediate "gray zone" where results are not conclusive. In both cohorts, the intermediate zone contained only 10% to 15% of participants. For those in the gray zone, a confirmatory PET scan or CSF test would still be needed.

This dual cut-off strategy resulted in 93% overall accuracy. Critically, less than 15–20% of patients fall into the ambiguous intermediate zone, which is the recommended maximum. This means the test can confidently classify the vast majority of people.

## Clinical Implications: What This Means for Patients

For a patient with memory complaints who is being evaluated for lecanemab or donanemab, this study suggests that a single blood test can replace the need for a lumbar puncture or PET scan in most cases.

If the blood test result falls clearly below the low cut-off, the patient is almost certainly Aβ negative and should not be given a DMT. If the result is clearly above the high cut-off, the patient is almost certainly Aβ positive and can proceed to treatment — after confirming eligibility based on other clinical factors (such as MMSE score and APOE status).

The sensitivity of 99% in the ITTC group is reassuring: fewer than 1 in 100 patients with true brain amyloid would be missed. The specificity of 87% means that about 13 in 100 people without amyloid might be told they are positive; however, the study authors note that most of these false positives would fall in an intermediate range, prompting further testing. Using the dual cut-offs, the number of people needing additional confirmation is kept to just 10–15%.

The study specifically validated these results in a cohort that matches the real population who would seek DMTs: people with MCI or mild AD, with MMSE scores of 22–28. This is exactly the group for which the drugs are approved.

Another practical advantage: the tests use the same Lumipulse platform already available in many clinical labs. The assay performance was rigorously validated, with inter-assay precision of 1.6% to 2.2% for high and low controls, far exceeding the required standards.

## Limitations: What This Study Couldn't Prove

This study has several limitations that patients and clinicians should consider.

First, the participants were all from the AIBL study in Australia, a research cohort that is predominantly White and of European ancestry. The authors note that there is insufficient evidence in under-represented ethnic populations, which can affect generalizability.

Second, the study used PET imaging as the reference standard. PET scans are imperfect at classifying borderline amyloid levels. Participants with CL values between 15 and 25 were deliberately excluded from the ADCC analysis, which may inflate the apparent test performance. The cutoff of ≥25 CL to define Aβ positivity also differs from some other studies that use ≥12 or ≥30.

Third, clinical classification within AIBL was based solely on neuropsychological test scores and a clinical panel decision, without using biomarker data. This is actually a strength for assessing the blood tests independently, but it also means the diagnostic categories rely on standard clinical judgment rather than a gold standard pathology.

Fourth, the bridging study between reagent lots was performed with a small sample (40 samples) that included only 10 from each diagnostic group. While the concordance correlations were high (0.93–0.99), some variability remains.

Fifth, the study does not tell us whether these blood tests can monitor a patient's response to DMT over time. The authors note this as a future direction: "When BBM data post-DMT become available, the question of whether these cut-offs can be used at a point where sufficient brain Aβ has been removed and treatment can be stopped needs to be addressed."

Finally, the statistical comparisons were generally not corrected for multiple testing. The authors acknowledge that some p-values between 0.002 and 0.05 should be considered "nominally significant" rather than definitive.

## Recommendations: What Patients Should Know

If you or a loved one is being evaluated for a disease-modifying Alzheimer's therapy, here are the key takeaways:

1.
**Ask about blood-based biomarker testing.** The combination of p-tau217 and the Aβ42/40 ratio, measured with Lumipulse, meets the international consensus criteria for a confirmatory test in populations similar to DMT candidates. This means it can replace CSF testing or PET in most cases.
1.
**Understand your numbers.** Your doctor will receive a value for the p-tau217/Aβ42 ratio. Low values (in the range of <0.006) are consistent with no brain amyloid; high values (above the upper cut-off, which the paper does not explicitly state but corresponds to roughly >0.03 on the transformed scale) indicate likely amyloid positivity. Many results will fall clearly on one side.
1.
**Know that a gray zone exists.** In 10–15% of cases, results are intermediate. If you fall in this zone, your doctor will recommend a PET scan or spinal fluid test to make the final decision. This is a small price to pay for avoiding unnecessary treatment.
1.
**The test is highly sensitive but not perfect.** A negative result (below the low cut-off) means you have a less than 1% chance of having significant brain amyloid, based on the ITTC sensitivity of 99%. A positive result means about an 87% chance you have amyloid — confirmed with a PET scan if needed.
1.
**Demographics don't change the story.** Adding age, sex, and APOE ε4 did not improve the predictive power of the p-tau217/Aβ42 ratio. So you don't need complex models to interpret your result.
1.
**Tests are reliable.** The Lumipulse platform used in this study showed very low variability between runs (coefficients of variation from 0.2% to 2.2%), meaning results are reproducible.

As always, talk to your healthcare provider about which tests are covered by your insurance and what they mean in your specific situation. The evidence is strong, but this is a rapidly evolving field — new cut-offs and recommendations may appear as more data emerge.

## Frequently Asked Questions

### What blood test can confirm brain amyloid buildup before starting Alzheimer's treatment?

A simple blood test measuring two markers – p-tau217 and the amyloid beta 42/40 ratio – can confirm brain amyloid buildup. In a study of nearly 400 Australians, this combination matched the accuracy of spinal fluid tests. If you're being evaluated for lecanemab or donanemab, ask your doctor if this blood test is available.

### Can this blood test replace the lumbar puncture or PET scan before I receive Alzheimer's treatment?

Yes, for most people. The combination test meets international criteria as a confirmatory test, so a spinal tap or PET scan is often unnecessary. However, in about 10–15% of results, the test falls into an intermediate gray zone, and then a PET scan or spinal fluid test is still needed to make the final decision.

### What happens if my blood test result is in the gray zone?

If your result is uncertain – not clearly positive or negative – doctors will recommend a PET brain scan or a lumbar puncture to confirm whether you have brain amyloid. Only about 10–15% of people get an inconclusive result, so most patients can avoid these extra procedures.

### What does a positive or negative result mean for getting lecanemab or donanemab?

A clearly negative result means you have less than a 1% chance of significant brain amyloid, so these drugs are unlikely to help. A clearly positive result means about 87% chance you have amyloid, and you may proceed with treatment if other eligibility criteria are met. Your doctor will interpret the exact number.

### Do these blood test results apply to people of all ethnic backgrounds?

The study participants were from an Australian research cohort, mostly of European ancestry. Researchers noted insufficient evidence in under-represented ethnic populations, which may affect generalizability. If you are from a different ethnic background, discuss with your doctor whether these test thresholds are appropriate for you.

### I had a p-tau217 and Aβ42/40 blood test that says I have brain amyloid. Should I get a second opinion before starting lecanemab or donanemab?

Before starting lecanemab or donanemab, a second opinion is worth seeking if your doctor's plan rests on a blood test alone. The combination of p-tau217 and the Aβ42/40 ratio has been shown to be a confirmatory test with over 90% accuracy against brain scans. In the population matching treatment candidates, sensitivity was 99% and specificity 87%. About 10–15% of results fall in an intermediate gray zone that still needs PET or spinal fluid testing. A second opinion can check whether your result is conclusive, whether additional confirmation is appropriate, and whether eligibility criteria such as MMSE score and APOE status are met. Diagnostic Detectives Network provides independent expert second opinions.

## Source Information

This patient-friendly article is based on peer-reviewed research.

Original article title: "Combining Lumipulse p-tau217 and Aβ42/40 as confirmatory tests for Aβ positivity prior to disease-modifying therapy"

Authors: James D. Doecke, Ahmed Chenna, Mintzu Lo, Youssouf Badal, Brandon Yee, Robert Martone, Christos Petropoulos, Christopher J. Fowler, Simon Laws, Stephanie R. Rainey-Smith, Ralph N. Martins, Christopher C. Rowe, Colin L. Masters, John Winslow

Publication: *Alzheimer's & Dementia*, 2025;21:e70707. DOI: 10.1002/alz.70707

Published under the Creative Commons Attribution-NonCommercial-NoDerivs License, © 2025 The Author(s).

This summary aims to make the research accessible while preserving all key findings, numbers, and caveats. It does not replace medical advice. Always consult a qualified clinician.


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