Can a Simple Blood Test Detect Alzheimer’s Disease?
Yes. A multinational study of 605 adults across six Latin American countries found that blood tests measuring three key proteins separated Alzheimer’s disease from healthy aging with an area under the curve of 83%, and frontotemporal lobar degeneration from healthy aging with an area under the curve of 88%. When those blood markers were combined with brain imaging and cognitive testing, the figures rose to 89% for Alzheimer’s and 95% for frontotemporal lobar degeneration.
Area under the curve is a measure of how well a test ranks a person with disease above a person without it. A perfect test scores 100%, and a coin flip scores 50%. It is not the same as the percentage of patients a test gets right in the clinic, which depends on where the cutoff is set and how common the disease is in the people being tested.
In Latin America, Alzheimer’s disease and frontotemporal lobar degeneration are diagnosed through clinical evaluation, neuropsychological tests, and structural brain imaging showing atrophy. That process is hindered by a shortage of trained specialists, limited access to medical care, and restricted availability of advanced imaging, which drives high rates of underdiagnosis and misdiagnosis. Blood-based markers are being validated for clinical use in the United States and Europe, but they have been studied far less in populations with the genetic, socioeconomic, and environmental diversity found across Latin America.
The researchers enrolled participants through the ReDLat consortium in Chile (147), Colombia (135), Mexico (104), Peru (102), Argentina (59), and Brazil (58). They measured amyloid-beta 42/40 ratios, which reflect the sticky plaques that build up in Alzheimer’s brains, phosphorylated tau (p-tau217 and p-tau181), which tracks abnormal tau protein, and neurofilament light chain, or NfL, which is a general marker of nerve cell injury.
What the Data Show
The amyloid-beta 42/40 ratio was lower in both the Alzheimer’s and the frontotemporal groups compared with cognitively normal participants. Phosphorylated tau and NfL were higher in both diseases. Within that pattern, p-tau217 was higher in Alzheimer’s disease and NfL was higher in frontotemporal lobar degeneration, which fits a disease that causes more widespread nerve damage. These differences held after adjusting for sex, age, and education.
Machine learning models trained on all four blood measures reached an area under the curve of 83% for Alzheimer’s versus cognitively normal, with p-tau217 the strongest single contributor, and 88% for frontotemporal lobar degeneration versus cognitively normal, with NfL the strongest contributor. Telling the two diseases apart from each other was harder: Alzheimer’s versus frontotemporal lobar degeneration reached only 72%.
The findings were consistent across countries in a meta-analysis, though heterogeneity between countries was high. Using the Youden index, the researchers identified cutoffs of 0.24 pg/mL for p-tau217 in distinguishing Alzheimer’s from normal cognition, and 21.91 pg/mL for NfL in distinguishing frontotemporal lobar degeneration from normal cognition. Higher NfL and p-tau217 were both linked to worse executive function and worse global cognition in both diseases, and p-tau217 was the only marker linked to memory in Alzheimer’s. Adding brain imaging and cognitive measures to the blood panel raised performance to 89% for Alzheimer’s and 95% for frontotemporal lobar degeneration.
Dr. Kumar’s Take
I find this study important for two reasons. First, it tests blood-based dementia markers in a population that biomarker research has largely left out. Ancestry analysis in this cohort confirmed a predominance of Amerindian backgrounds. Second, the practical implications are real. If a blood draw can do much of the work that currently requires advanced imaging, early diagnosis becomes possible in clinics and communities that have no access to that equipment.
I would be careful about how far to push it. Blood markers alone separated each disease from healthy aging reasonably well, but they did a poor job of separating Alzheimer’s from frontotemporal lobar degeneration, and that is exactly the distinction a clinician struggles with when a patient presents with overlapping cognitive and behavioral symptoms. Up to 28.6% of patients meeting clinical criteria for behavioral variant frontotemporal dementia show biomarker profiles that look like Alzheimer’s. The best performance in this study came from combining blood markers with imaging and cognitive testing, not from replacing them. That is how I would use these tests today: as a way to decide who needs a full workup, not as the workup itself.
How These Blood Tests Work
Think of these blood markers as three different alarm signals. The first, the amyloid-beta 42/40 ratio, compares two forms of a protein that clumps together in Alzheimer’s brains. When the ratio drops, it suggests amyloid plaques are forming. The second, phosphorylated tau (p-tau217 and p-tau181), rises when brain cells accumulate abnormal tau protein, another hallmark of Alzheimer’s. The third, neurofilament light chain, leaks into the blood when nerve cells are damaged or dying, which is why it rose most in frontotemporal lobar degeneration. This is the AT(N) framework: amyloid, tau, and neurodegeneration. Together, the three signals give a biological picture of what is happening inside the brain from a single blood sample.
Who Benefits Most
Dementia affects 8.5% of people aged 60 and older in Latin America, one of the highest rates in the world, and that figure is projected to reach 19.3% by 2050, or roughly 12 million cases. Diagnosis often comes late, when options are more limited. A test that runs on a standard blood draw could help primary care doctors identify who needs further evaluation earlier, in settings where specialists and advanced imaging are scarce. The 2024 Alzheimer’s Association guidelines already define Alzheimer’s as a biological process and incorporate blood-based markers into staging and clinical assessment. The study also matters for research, since trials need affordable ways to identify and enroll the right patients.
Practical Takeaways
- If you or a family member are concerned about memory changes, ask your doctor about blood-based biomarker tests, which may help guide next steps without invasive procedures.
- No single blood test can confirm or rule out Alzheimer’s on its own. In this study the highest performance came from combining blood markers with brain imaging and cognitive testing, so a full evaluation still matters.
- Blood markers were much better at separating each disease from healthy aging than at separating Alzheimer’s from frontotemporal dementia, so a specialist evaluation remains necessary when the clinical picture is ambiguous.
Related Studies and Research
These findings connect to a growing body of research on brain proteins, sleep, and neurodegeneration:
- One night of sleep loss increases Alzheimer’s protein in spinal fluid, showing how sleep disruption raises the same amyloid-beta proteins measured in this blood test study.
- One night without sleep increases Alzheimer’s protein in the brain, which found that even a single sleepless night can boost amyloid buildup.
- The sleep-wake cycle controls tau protein clearance in brain and spinal fluid, highlighting how sleep helps clear the tau protein that this study measured in blood.
- Early Alzheimer’s at 50: how TMS helped one patient’s memory, exploring treatment options once early-onset Alzheimer’s is detected.
FAQs
How well do these blood tests detect Alzheimer’s disease?
In this study, the blood panel alone reached an area under the curve of 83% for separating Alzheimer’s disease from normal cognition. When the researchers added brain imaging and cognitive testing, that figure climbed to 89%. Area under the curve describes how reliably a test ranks a person with the disease above a person without it, so it is a measure of discrimination rather than a count of correct diagnoses. The appeal of a blood test is that it needs only a blood draw and basic lab access, which makes it usable as a first step in places where imaging is not available.
What is frontotemporal dementia, and how is it different from Alzheimer’s?
Frontotemporal lobar degeneration affects behavior and language, and its early presentation can overlap with early Alzheimer’s disease, which is part of why the two are so often confused. In this study, participants with frontotemporal lobar degeneration were younger on average (67.2 years) than those with Alzheimer’s disease (70.6 years), and they had the highest scores for functional impairment and neuropsychiatric symptoms of any group. The blood markers separated frontotemporal lobar degeneration from normal cognition slightly better than they did Alzheimer’s (88% versus 83%), largely because neurofilament light chain rises more in frontotemporal disease. Distinguishing the two diseases from each other with blood alone was much weaker, at 72%.
When will blood tests for Alzheimer’s be available to the general public?
These markers are currently being validated for clinical use in the United States and Europe, where they have been studied extensively, and the 2024 Alzheimer’s Association guidelines already incorporate blood-based markers into staging and clinical assessment. Their validation in more diverse populations has lagged behind, which is what this Latin American study begins to address. I would not expect a blood test to stand alone as a diagnosis. The evidence here points toward blood markers as one part of an evaluation that also includes cognitive testing and imaging.
Bottom Line
A blood panel measuring amyloid-beta ratios, phosphorylated tau, and neurofilament light chain separated Alzheimer’s disease from normal cognition with an area under the curve of 83%, and frontotemporal lobar degeneration with 88%, across 605 participants in six Latin American countries. Adding brain imaging and cognitive testing raised those figures to 89% and 95%. Separating the two diseases from each other with blood alone was considerably weaker, at 72%. The practical value is in extending early evaluation to populations and regions where advanced imaging is out of reach.
