How accurate is the blood test compared to the standard spinal fluid test?
The most promising blood test measures a specific form of tau protein called p-tau217. In a large study of over 1,700 people across two countries, this blood test performed just as well as the FDA-approved spinal fluid tests for detecting Alzheimer's brain plaques. The test's accuracy was measured by the area under the curve (AUC), where 1.0 is perfect. The blood test scored 0.95 to 0.97, which is clinically equivalent to the spinal fluid tests [1][2]. For detecting tau tangles (another hallmark of Alzheimer's), the blood test was actually superior, with AUCs of 0.95 to 0.98 [1][2].
In practical terms, for people with cognitive symptoms, the blood test correctly identified Alzheimer's pathology 89-90% of the time. When the test result was borderline, a two-cutoff approach (where results are reported as positive, negative, or intermediate) pushed the accuracy up to 95% [1][2]. This means the blood test is reliable enough to be used in specialist memory clinics, reducing the need for more invasive and expensive spinal taps or brain scans.
Are all blood tests for Alzheimer's equally good?
No, not all blood tests are created equal. A head-to-head comparison of six leading commercial blood tests found that measures of p-tau217 were the most accurate for classifying both amyloid plaques and tau tangles [7]. In contrast, the blood test measuring the ratio of amyloid-beta 42 to 40 (Aβ42/Aβ40) had relatively low accuracy for detecting amyloid plaques [7]. Other blood markers, like neurofilament light chain (NfL), are useful for tracking neurodegeneration but are not specific to Alzheimer's [6][7].
The key takeaway is that the p-tau217 test is the standout performer. It consistently outperforms other blood markers and matches or beats spinal fluid tests. However, even the best blood test is not perfect. Its accuracy depends on the population being tested. In a memory clinic where many patients have Alzheimer's, a positive result is very reliable. But if the test were used in a low-risk population (like healthy young adults), the chance of a false positive would be much higher [5]. That's why current expert guidelines recommend using these blood tests only in people with cognitive symptoms, and only in specialist settings [8][9].
Can a blood test replace a brain scan or spinal tap?
For many patients, yes, a blood test can replace the need for a spinal tap or a brain scan. The 2024 NIA-AA research guidelines now include blood biomarkers as a core diagnostic tool, stating that accurate plasma biomarkers can confirm a diagnosis of Alzheimer's [4]. However, the guidelines also say that if a blood test is abnormal, it should ideally be confirmed with a spinal fluid test or PET scan [4][8].
In practice, the blood test is already being used as a prescreening tool for clinical trials, and it is starting to be used in specialist memory clinics to reduce the number of spinal taps and scans needed [8][9]. For example, the p-tau217 blood test can identify people with intermediate or advanced Alzheimer's stages with 92% accuracy, which could cut the number of expensive tau PET scans required by half [3]. So while the blood test is not yet a standalone diagnostic for every situation, it is powerful enough to significantly streamline the diagnostic process and make it more accessible.
About These Sources
This answer is built on 9 peer-reviewed studies — published from 2022 to 2025, 6 from 2024 or later, 8 in Q1 journals, collectively cited 1,464 times — selected as the most relevant from 11 studies that passed quality screening, drawn from 57 papers retrieved from a database of over 500 million.
Sources used in this answer
Highly accurate blood test for Alzheimer’s disease is similar or superior to clinical cerebrospinal fluid tests
In a study of 1,759 people across two cohorts, the plasma %p-tau217 blood test was clinically equivalent to FDA-approved CSF tests for detecting amyloid plaques (AUC 0.95-0.97) and superior for detecting tau tangles (AUC 0.95-0.98). In cognitively impaired individuals, accuracy was 89-90%, rising to 95% with a two-cutoff approach.
A highly accurate blood test for Alzheimer’s disease pathology has performance equivalent or superior to clinically used CSF tests
This companion paper to [1] confirms that plasma %p-tau217 performs equivalently or superiorly to FDA-approved CSF tests for detecting Alzheimer's pathology, with accuracy, positive predictive value, and negative predictive value of 89-90% for amyloid PET and 87-88% for tau PET in cognitively impaired subcohorts.
Alzheimer’s disease biological PET staging using plasma p217+tau
In 475 participants, plasma p217+tau discriminated combined Intermediate/Advanced Alzheimer's stages from no Alzheimer's with AUC 0.92, sensitivity 0.77, and specificity 0.91. It was less effective at distinguishing Advanced stage alone (PPV only 0.33), but could reduce the need for tau PET scans.
Biomarkers in Alzheimer’s disease: Emerging trends and clinical implications
This review highlights that plasma p-tau217 has AUCs of 0.95-0.98 for Alzheimer's pathology, comparable to or surpassing CSF tests. It notes that the 2024 NIA-AA guidelines now include accurate plasma biomarkers as core diagnostic tools, but standardized cutoffs are still needed.
Blood tests for Alzheimer's disease: The impact of disease prevalence on test performance
This commentary emphasizes that the diagnostic performance of Alzheimer's blood tests depends heavily on disease prevalence in the tested population. A test with 90% sensitivity and 90% specificity can have a positive predictive value as low as 60% in a low-prevalence setting, warning against use in healthy or young populations.
Diagnostic Blood Biomarkers in Alzheimer’s Disease
In 136 participants across the Alzheimer's spectrum, plasma NFL/Aβ1-42 ratio correlated strongly with CSF measures (r=0.558) and hippocampal volume (r=0.409), and showed higher diagnostic accuracy than individual biomarkers in early Alzheimer's.
Head‐to‐head comparison of leading blood tests for Alzheimer's disease pathology
In a head-to-head comparison of six commercial blood tests using Alzheimer's Disease Neuroimaging Initiative samples, plasma p-tau217 measures (alone or combined) had the strongest relationships with amyloid PET, tau PET, and cortical thickness, while plasma Aβ42/Aβ40 had relatively low accuracy for amyloid status.
The Alzheimer's Association appropriate use recommendations for blood biomarkers in Alzheimer's disease
The Alzheimer's Association appropriate use recommendations state that blood biomarkers can be used as prescreeners for clinical trials (with confirmation by PET or CSF) and cautiously in specialist memory clinics for patients with cognitive symptoms, but not yet as standalone diagnostics or in primary care.
Blood biomarkers for Alzheimer’s disease in clinical practice and trials
This review concludes that plasma p-tau assays have high diagnostic accuracy for distinguishing Alzheimer's from other neurodegenerative diseases in cognitively impaired patients, and can reduce the need for CSF or PET in specialist memory clinics. They are already used to identify presymptomatic Alzheimer's in trials.