ATN Profile: For and beyond Alzheimer’s disease

Alzheimer’s disease (AD) is the most common form of dementia, accounting for 60%-70% of dementia cases.¹ Blood-based biomarkers (BBMs) specific to AD pathology, such as beta-amyloid 42/40 and pTau217, are now available to clinicians. But this still leaves up to 40% of patients with clinical signs of mild cognitive impairment (MCI) or possible dementia without answers.

With new therapies available for AD, appropriate triaging of patients who need a neurologist for treatment and monitoring is critical. BBMs can help with that triaging process, and pTau217 has emerged as a lead candidate for a triaging tool. Indeed, a single biomarker with high sensitivity and specificity to rule-in patients likely of having AD is promising. However, using this marker alone leaves the other 30%-40% of patients, leaving clinicians with little else to go on.

When a patient with possible MCI or dementia first presents, the primary clinical question for the clinician is, “What is going on with this patient?” The question of whether a patient may have AD is a secondary clinical question, alongside questions regarding polypharmacy, vitamin deficiencies, hormonal imbalances or metabolic problems. Using pTau217 only addresses one of the many secondary clinical questions, making it a less effective clinical approach when compared to a BBM panel, which provides additional insight and evidence toward the primary clinical question.

This was the impetus behind Labcorp’s ATN Profile, which employs three well-studied BBMs in a single assay. Designed to help rule-in or rule-out evidence of AD pathology, the profile also provides additional biological evidence to help guide clinicians for those 30%-40% of patients who do not have AD. The three biomarkers in the ATN Profile are:

• A: Plasma Aβ42/40 immunoassay. Ovod et al² demonstrated that accurate amyloid-beta assessments for AD from blood were possible using mass spectrometry techniques. More recently, studies have demonstrated that plasma Aβ42/40 assessments could achieve levels similar to mass spectrometry-based methods for detecting AD pathology.^3,4 Labcorp’s own internal validation of this Aβ42/40 immunoassay measured 200 plasma specimens acquired from the Australian Imaging, Biomarker and Lifestyle (AIBL) Study of Ageing.⁵ Specimens were obtained from amyloid PET-negative subjects classified as cognitively unimpaired and amyloid PET-positive subjects classified as having no cognitive impairment, MCI or AD dementia. These same samples were run through Labcorp’s pTau217 assay. Performance for the Aβ42/40 immunoassays was nearly equivalent to pTau217 (Figure 1). Receiver operating characteristic (ROC) analysis of clinical specimen results from validated assays produced an area under the curve (AUC) of 0.94 for both Aβ42/40 and pTau217. The sensitivity (96%) and specificity (87%) observed for Aβ42/40 measurements meets current recommendations for triage testing and was slightly better, although not statistically significant, compared to pTau217 (Figure 1). Notably, the same Aβ42/40 measurement system and assays utilized herein produced similar AUC results reported for different clinical cohorts with the same measurement system, making this assay one of the best-performing Aβ42/40 immunoassay to be validated with two independent cohorts on two different continents.^3,4,6 
• T: Plasma pTau181 immunoassay. pTau181 was discovered prior to pTau217. In time, pTau217 has been demonstrated to be the better phosphorylated tau marker for AD because it serves as a specific surrogate for beta-amyloid pathology with high correlation to PET. Given the performance of the Aβ42/40 assay and the design goals of the ATN Profile, a second high-precision marker for detecting amyloid pathology was not necessary. pTau181 is used intentionally because it may help differentiate between AD and suspected non-AD pathophysiology (SNAP), which would rule a patient out of anti-amyloid therapy.⁷ Elevated pTau181 levels, in the absence of evidence of beta-amyloid pathology, is associated with memory impairments and atrophy in the medial temporal lobe.⁸ For an initial BBM-based triaging test that goes beyond the binary question of AD or not, pTau181 may be a more useful marker.
• N: Plasma neurofilament light chain (NfL). NfL is an indicator of neurodegeneration but is not specific to any particular disease, as neurofilaments in blood are simply the result of axonal damage, regardless of cause. For AD, NfL can be useful as an indicator of disease progression and severity.⁹ In the absence of beta-amyloid pathology, higher NfL levels can be indicative of other neurodegenerative diseases that a physician should investigate, particularly if pTau181 is elevated.

Interpretation

Hampel et al¹⁰ published a review article summarizing how to interpret and understand the eight possible outcomes from an ATN-like test. Each assay within the ATN panel has a cutoff that provides an indication of whether a patient’s measured value is considered normal or abnormal. For NfL, cutoffs are based on age ranges recognizing that baseline measurable NfL levels increase with age.¹¹ Each of the three assays is then given a “-” or “+” indicator corresponding to a normal or abnormal result, respectively. There are eight possible combinations of results, and these combinations group into three possible clinical interpretations: normal, AD continuum (“AD pathology” and “probable AD”) and non-AD pathology or neurodegenerative disease.

A normal result, where there is no evidence of disease, occurs when each of the three markers is negative: A-, T- and N-. Any result where A is positive indicates that the patient is on the AD continuum. If these patients are not already under the care of a neurologist, they are candidates for immediate referral, where these findings can then be confirmed and potential treatment options can be assessed. Any result where A is negative, but T and/or N are positive, indicates non-AD pathology or neurodegeneration. This additional set of A negative results provides significant value to a clinician trying to address the primary clinical question of what might be going on with the patient.

Clinical use for patient triaging

Based on the demonstrated performance of the plasma Aβ42/40 component of the ATN Profile, the assay meets the newly published recommendations for sensitivity (≥90.0%) and specificity (≥85.0%) for a triage test performed in a primary care setting.¹² For neurologists familiar with the various markers and the type of information they convey about disease, pTau217 is a simple single marker likely to quickly become the default assay for assessing AD. However, in primary care settings, the ATN Profile has notable benefits over pTau217. Providing important clinical information about the patient’s health beyond just AD makes this a more useful assay for assessment and triage. In terms of interpretation, primary care physicians are no more acquainted with pTau217 than they are with any of the ATN Profile components. The results of the three biomarkers are summarized using language that abstracts the complexity of the individual biomarker results. Ultimately, the ATN Profile provides the physician with a clear summary of likely AD status and whether there is evidence of other non-AD pathologies that should be investigated.

References

1. Dementia. World Health Organization. March 15, 2023. Accessed August 19, 2024. https://www.who.int/news-room/fact-sheets/detail/dementia
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3. Yamashita K, Miura M, Watanabe S, et al. Fully automated and highly specific plasma β-amyloid immunoassays predict β-amyloid status defined by amyloid positron emission tomography with high accuracy. Alzheimers Res Ther. 2022;14(1):86. doi:10.1186/s13195-022-01029-0
4. Yamashita K, Watanabe S, Ishiki K, et al. Fully automated chemiluminescence enzyme immunoassays showing high correlation with immunoprecipitation mass spectrometry assays for β-amyloid (1-40) and (1-42) in plasma samples. Biochem Biophys Res Commun. 2021;576:22-26. doi:10.1016/j.bbrc.2021.08.066
5. Fowler C, Rainey-Smith SR, Bird S, et al. Fifteen years of the Australian Imaging, Biomarkers and Lifestyle (AIBL) Study: progress and observations from 2,359 older adults spanning the spectrum from cognitive normality to Alzheimer’s disease. J Alzheimers Dis Rep. 2021;5(1):443-468. doi:10.3233/ADR-210005
6. Bun S, Ito D, Tezuka T, et al. Performance of plasma Aβ42/40, measured using a fully automated immunoassay, across a broad patient population in identifying amyloid status. Alzheimers Res Ther. 2023;15(1):149. doi:10.1186/s13195-023-01296-5
7. Oberstein TJ, Schmidt MA, Florvaag A, et al. Amyloid-β levels and cognitive trajectories in non-demented pTau181-positive subjects without amyloidopathy. Brain. 2022;145(11):4032-4041. doi:10.1093/brain/awac297
8. Jack CR. PART and SNAP. Acta Neuropathol. 2014;128(6):773-776. doi:10.1007/s00401-014-1362-3
9. Janelidze S, Palmqvist S, Leuzy A, et al. Detecting amyloid positivity in early Alzheimer's disease using combinations of plasma Aβ42/Aβ40 and p-tau. Alzheimers Dement. 2022;18(2):283-293. doi:10.1002/alz.12395
10. Hampel H, Cummings J, Blennow K, Gao P, Jack CR, Vergallo A. Developing the ATX(N) classification for use across the Alzheimer disease continuum. Nat Rev Neurol. 2021;17(9):580-589. doi:10.1038/s41582-021-00520-w
11. Khalil M, Pirpamer L, Hofer E, et al. Serum neurofilament light levels in normal aging and their association with morphologic brain changes. Nat Commun. 2020;11(1):812. doi:10.1038/s41467-020-14612-6
12. Schindler SE, Galasko D, Pereira AC, et al. Acceptable performance of blood biomarker tests of amyloid pathology — recommendations from the Global CEO Initiative on Alzheimer’s Disease. Nat Rev Neurol. 2024;20:426-439. doi:10.1038/s41582-024-00977-5