Clinical Biomarker Testing for Alzheimer Disease Pathology in the Context of Amyloid-Targeting Treatments

Cognitive impairment is a frequent concern in older adults and has a wide variety of causes.^1,2 In older adults with cognitive impairment, the most common cause is Alzheimer disease (AD), which is defined neuropathologically by amyloid-β (Aβ) plaques and tau neurofibrillary tangles.³ Even after a comprehensive clinical evaluation, symptomatic AD (defined as either mild cognitive impairment or dementia due to AD) is often misdiagnosed unless biomarker testing is performed to determine the presence of AD pathology.⁴

Until recently, biomarker testing for AD pathology was performed only occasionally and almost exclusively by memory specialists. Clinical testing typically involved cerebrospinal fluid (CSF) tests or, rarely, amyloid PET scans in cases with high diagnostic uncertainty. With the approval of amyloid-targeting treatments (ATTs) that slow cognitive decline in people with early symptomatic AD (ie, mild cognitive impairment or mild dementia due to AD), biomarker testing has become increasingly important, not only to improve diagnostic accuracy but also to determine whether individuals are candidates for these therapies.^5,6 In parallel, blood tests for AD pathology have developed rapidly over the past 5 years and are increasingly being used by memory specialists, general neurologists, and primary care providers (PCPs) in the assessment of individuals with cognitive impairment.^7,8

When used appropriately and integrated with a comprehensive clinical assessment, AD biomarker tests have the potential to improve early and accurate diagnosis of symptomatic AD, identify suitable candidates for AD-specific treatments, and monitor the effects of treatments (Figure).⁹

Figure. Relevance of different Alzheimer disease (AD) biomarker modalities for diagnosis and monitoring in the era of amyloid-targeting treatments (ATTs). 
Abbreviations: CSF, cerebrospinal fluid; PET, positron emission tomography.

Clinical Assessment of Cognitive Impairment 

Cognitive concerns are common in older adults and may result from treatable medical causes (eg, medication side effects, mood or sleep disorders, cerebrovascular disease, metabolic abnormalities) or neurodegenerative diseases (eg, AD, dementia with Lewy bodies).⁹ Especially given that cognitive impairment frequently has a multifactorial etiology, a comprehensive assessment is essential to ensure that all major contributors are identified and addressed. 

The initial workup for cognitive impairment includes a detailed history of concerns (eg, cognitive, behavioral, motor, and functional changes), medical history, medications, family history, social history, general and neurologic examination, cognitive testing, routine bloodwork, and structural brain imaging.² Clinical features that support a diagnosis of symptomatic AD include slowly progressive cognitive decline lasting ≥6 months, which often initially affects episodic memory.⁹ AD biomarker testing may be indicated if the patient has objective evidence of cognitive impairment, symptomatic AD is a possible diagnosis, and the patient and clinician agree that biomarker testing would be beneficial to care.^7,10,11

The patient, caregiver, and clinician should consider the optimal biomarker testing modality based on patient characteristics and practical factors, including medical comorbidities (such as chronic kidney disease [CKD], which may affect blood test results), test-related issues (eg, insurance requirements for CSF testing or amyloid PET to qualify for coverage of ATTs), and cost.⁹ The clinician should clarify that the purpose of AD biomarker testing is not to determine whether cognitive impairment is present, which is instead measured by cognitive testing, but whether AD pathology is present and potentially causing or contributing to cognitive symptoms. It’s important to note that positive AD biomarkers do not establish that AD is the primary cause of cognitive symptoms, particularly in older individuals with mixed pathologies. An accurate diagnosis requires integration of the comprehensive clinical assessment and AD biomarker test results.

 AD biomarker testing should generally be reserved for individuals with objectively documented cognitive impairment in whom AD is a diagnostic consideration and test results are expected to influence clinical management. Biomarker testing is not currently recommended as a screening tool in cognitively unimpaired individuals in routine clinical practice. Before testing, clinicians should discuss the potential benefits, limitations, and implications of biomarker results with patients and care partners.

CSF Tests

CSF tests were the first tests used clinically for AD pathology¹² and the dominant biomarker modality used for clinical testing until recently. CSF biomarker levels represent the balance of production and clearance of specific analytes at the time the CSF was collected. People with AD pathology typically have lower CSF levels of Aβ42 and higher levels of total tau and phosphorylated tau 181 (p-tau181). However, the most accurate measure of amyloid pathology is not CSF Aβ42. Instead, the ratios of Aβ42/40, total tau/Aβ42, and p-tau181/Aβ42 better classify amyloid status and are the US Food and Drug Administration (FDA)–cleared measures recommended for diagnosis of AD pathology.¹³ Some FDA-cleared CSF tests have been validated against amyloid PET and agree with PET status in ~90% of individuals.¹³

CSF tests may be especially useful in individuals with possible non–AD causes of cognitive impairment for which specific CSF tests are available, such as dementia with Lewy bodies, Creutzfeldt-Jakob disease, or autoimmune-mediated encephalopathy.⁹ CSF testing is not optimal for some individuals: it is contraindicated in individuals receiving anticoagulation and results may be affected by central nervous system disorders affecting CSF circulation, such as normal-pressure hydrocephalus.^9,14 More common drawbacks include that many individuals perceive lumbar puncture as invasive and that lumbar puncture may be burdensome to clinicians.

Amyloid PET

Amyloid PET has been widely used in research studies since the early 2000s, with one radiotracer receiving FDA approval for clinical diagnosis of AD pathology in 2012 after demonstrating high concordance with postmortem autopsy results.¹⁵ Amyloid PET enables the visualization and quantification of the burden of amyloid plaques accumulated over the lifetime. Clinical amyloid PET scans have typically been read visually as either positive or negative; however, quantitation of amyloid burden using computerized algorithms is increasingly being performed as an adjunct to visual interpretation. The amyloid PET visual read and quantitative value agree in ~86% of cases in real-world settings.¹⁶ However, amyloid PET imaging is expensive, involves the use of radioactive tracers, and requires highly specialized equipment and personnel, limiting its widespread use.¹¹

Blood Tests

Recent technical and scientific advances have yielded accurate blood tests for AD pathology that are clinically available in the United States and other countries (Table).^7,13 Information on clinically available blood tests for AD is rapidly expanding and evolving. An updated summary of tests is available from the Global CEO Initiative on Alzheimer’s Disease (https://alzdiagnostichub.org/database).¹⁷ Similar to CSF tests, blood biomarker levels represent the balance of production and clearance of specific analytes at the time the blood was collected. 

The most accurate clinical tests for AD pathology include plasma measures of phosphorylated tau 217 (p-tau217) or the p-tau217/non–p-tau217 ratio.^18,19 Per the recommendations of the Alzheimer’s Association Clinical Practice Guidelines and the Global CEO Initiative on Alzheimer’s Disease, blood tests with ≥90% sensitivity and ≥75% specificity can be used for triaging, with a positive result followed by a second test.^7,13 Blood tests with ≥90% sensitivity and ≥90% specificity have accuracy similar to CSF tests or PET scans and therefore can be used to confirm amyloid pathology without a second test.^7,13 Plasma p-tau217–based tests that apply a single cutoff approach achieve a sensitivity and specificity approaching 90%.^13,18-20 The sensitivities and specificities of these tests may improve to >90% when a 2-cutoff approach is applied, although this results in ~15% to 20% of individuals having intermediate values that require further testing.^13,21,22

In May 2025, the Lumipulse plasma p-tau217/Aβ42 test (Fujirebio, Malvern, PA) became the first blood-based test to receive FDA clearance to aid in the diagnosis of AD in adults ≥55 years with cognitive impairment. Subsequently, the Elecsys p-tau181 plasma test (Roche, Indianapolis, IN) received FDA clearance for triaging individuals with cognitive concerns in primary care. Several additional p-tau217–based tests are available as laboratory-developed tests and are undergoing review by the FDA (https://alzdiagnostichub.org/database).¹⁷ Some tests provide amyloid probability scores that reflect the combined effects of multiple analytes and facilitate interpretation of results.

Blood tests for AD have major practical advantages compared with CSF tests and amyloid PET scans. Compared with CSF tests and amyloid PET scans, blood tests have characteristics that are ideally suited to widespread use (eg, higher acceptability and accessibility, lower burden, lower cost, improved scalability).^13,23 However, some obstacles remain. There is inconsistent insurance reimbursement for blood tests for AD, and only 2 of the many clinically available tests have received FDA clearance. Blood tests for AD may be affected by some medical conditions, such as CKD,²⁴ although clinicians may be able to readily identify CKD through routine blood tests. Furthermore, substantial variability remains in the accuracy of clinically available blood tests for AD.^18,19 Therefore, clinicians must be well-informed to appropriately select and interpret blood tests for AD. The rapid refinement of tests and numerous publications demonstrating high accuracy of p-tau217–based tests is accelerating the clinical implementation of these highly accurate tests.

Biomarker Testing in the Context of ATTs

Recent increases in AD biomarker testing are partly driven by the availability of ATTs, which are indicated only for individuals with early symptomatic AD.⁸ These treatments are thought to be most effective earlier in the disease course, making the diagnosis of AD time-sensitive. Especially given the limited number of memory clinics and the large population of older adults with memory concerns, providing an early diagnosis to more individuals will require PCPs to identify individuals likely to have symptomatic AD.²³ Because CSF tests and amyloid PET are rarely available outside of memory clinics, PCPs will increasingly use blood tests for AD to assist in the timely diagnosis of individuals with possible symptomatic AD who can then be referred for further specialist evaluation and potential treatment.²³

Initiation of ATTs requires a diagnosis of early symptomatic AD and biomarker confirmation of amyloid pathology. US Medicare coverage in some regions allows initiation of ATTs based on blood tests for AD.²⁵ However, some Medicare Advantage plans and private insurers require positive CSF tests or amyloid PET scans. Therefore, if a patient is a potential candidate for ATTs, clinicians should choose a biomarker modality that will enable treatment. 

Only amyloid PET can be used to monitor for clearance of amyloid pathology by ATTs, and there is no evidence that CSF tests or blood tests can establish amyloid clearance.²⁶ Some providers prefer to perform an amyloid PET scan to confirm amyloid status in potential candidates for ATTs so that this pretreatment baseline PET scan can be compared with future scans to assess the extent of amyloid clearance. Although there may be interest in the extent of amyloid clearance by Leqembi (lecanemab; Eisai, Nutley, NJ) at 18 months, an amyloid PET scan is not required, and most patients transition to maintenance therapy.²⁶ For patients treated with Kisunla (donanemab; Eli Lilly, Indianapolis, IN), a follow-up amyloid PET scan at 12 or 18 months is likely to affect the decision about when to stop treatment.⁶ However, this decision depends on the results of the amyloid PET scan performed at 12 or 18 months (or later), not on the results of the baseline amyloid PET scan.

The baseline amyloid burden may theoretically help clinicians estimate the expected length of treatment until clearance, but the timing of follow-up amyloid PET scans is typically decided by factors such as local protocols or insurance requirements. Overall, a baseline amyloid PET scan may have some value in patients receiving donanemab treatment but is not essential, whereas follow-up amyloid PET scans are often indicated for determining when donanemab can be stopped. 

Conclusions

Cognitive impairment in older adults is common and can have numerous and multifactorial etiologies. Biomarker testing is essential to determine whether AD pathology is present and may be causing or contributing to cognitive impairment; however, biomarker results must be integrated with a comprehensive clinical assessment to provide an accurate diagnosis. Providing an earlier diagnosis will likely require PCPs to use blood-based tests for AD to diagnose individuals with possible early symptomatic AD. Depending on the initial workup and practical factors, memory specialists may need to perform further evaluation and potentially confirm amyloid pathology with additional biomarker tests before considering ATTs. 

Providers must consider patient characteristics and practical factors when determining which biomarker test to order. CSF tests are especially helpful in individuals with possible non–AD conditions for which specific CSF tests are available. Amyloid PET scans are the only modality that can be used to evaluate for clearance of amyloid by ATTs. Appropriate use of AD biomarker tests and integration with the clinical assessment will enable individuals to receive an earlier and more accurate diagnosis and more timely access to treatment.