Memory Digest

Biomarkers

Biomarkers are measurable signs of disease in the body, such as amyloid and tau, that can be seen on brain scans, in spinal fluid or in blood; learn what they are, how they help diagnose Alzheimer's, and their limits.

Last reviewed: October 2026

You've probably heard that doctors can now "see" Alzheimer's with a scan or a blood test. What they're measuring are biomarkers: short for biological markers, measurable signs in the body that show a disease process is happening.

Biomarkers are part of everyday medicine. Blood pressure is a biomarker for heart disease risk; blood sugar is a biomarker for diabetes. In dementia, biomarkers have changed both research and diagnosis in just a few years.

Which biomarkers matter in Alzheimer's?

Alzheimer's disease is defined by two abnormal proteins in the brain (see Amyloid and Tau Explained):

• Amyloid, which clumps into plaques between nerve cells
• Tau, which forms tangles inside nerve cells

Other biomarkers show nerve cell damage or loss (for example, shrinkage on an MRI or a protein called neurofilament light) or inflammation.

According to the National Institute on Aging (NIA), these changes can begin years before symptoms appear, which is why biomarkers are so useful in research.

How biomarkers are measured

Brain scans (PET). A PET scan uses a small amount of a radioactive tracer that sticks to amyloid or tau, so doctors can see where they've built up. See Brain Imaging: MRI, CT and PET.

Spinal fluid (CSF). A lumbar puncture collects a small sample of the fluid around the brain and spinal cord, which can be tested for amyloid and tau. See Spinal Fluid (CSF) Testing.

Blood tests. The newest option. In May 2025 the FDA cleared the first blood test to help diagnose Alzheimer's. It measures a form of tau called p-tau217 and a ratio with amyloid, for adults 55 and older who already have signs and symptoms. More blood tests have been cleared since. See Blood Tests for Alzheimer's.

MRI and CT don't show amyloid or tau, but they can show shrinkage, strokes and other causes of symptoms.

How biomarkers help people today

• Making a more accurate diagnosis. Symptoms alone can't always tell Alzheimer's apart from other dementias. Biomarkers can confirm whether Alzheimer's changes are present.
• Deciding on treatment. The anti-amyloid drugs Lecanemab (Leqembi) and Donanemab (Kisunla) are only for people with confirmed amyloid, so a PET scan or spinal fluid test is required.
• Ruling things out. A negative result can point doctors toward other causes, some of them treatable.
• Joining research. Many Joining a Clinical Trial use biomarkers to find the right participants and to measure whether a drug is working.

A new way of defining Alzheimer's

In 2024, an Alzheimer's Association workgroup published revised criteria that define Alzheimer's disease biologically, by the presence of key biomarkers, rather than only by symptoms. It's a major shift. Under this view, a person can have Alzheimer's brain changes before memory problems appear.

The same workgroup also said that, for now, these criteria are mainly meant to guide research and clinical care for people with symptoms. Testing people without symptoms is not recommended outside research at this time. Not everyone with amyloid in the brain goes on to develop dementia, and there's no approved treatment for people without symptoms.

Limits to keep in mind

• A positive biomarker isn't a crystal ball. It shows that Alzheimer's changes are present. It can't say exactly when, or whether, symptoms will develop.
• Results need context. Many older adults have some amyloid; many people have more than one brain condition at once (see Mixed Dementia).
• Blood tests are not all the same. Accuracy varies by test and lab. A clinician should order and interpret them, and some results need confirmation.
• Biomarkers for other dementias like Lewy body and frontotemporal dementia are still being developed.
• Cost and coverage vary (see What Testing Costs and Insurance Covers).

Questions to ask your doctor

1. What will this test tell us, and how will it change our plan?
2. How accurate is this particular test for someone like me?
3. What happens if it's positive? If it's negative?
4. Will insurance cover it?
5. Should I talk with a genetic counselor or join a research study?

Where biomarker research is heading

Scientists are working on blood tests for screening in primary care, markers for Lewy body and other dementias, and ways to track whether treatments are working. Many prevention trials now enroll people based on biomarkers alone (see Prevention Trials).

For more on how memory problems are evaluated, see Getting a Diagnosis.

Sources

1. National Institute on Aging: How Biomarkers Help Diagnose Dementia
2. Jack CR Jr et al. Revised criteria for diagnosis and staging of Alzheimer's disease: Alzheimer's Association Workgroup. Alzheimer's & Dementia, 2024
3. FDA: FDA Clears First Blood Test Used in Diagnosing Alzheimer's Disease (May 2025)

Not medical advice. This article is general information, not a diagnosis or treatment plan for any individual. Talk with a doctor about your situation.

Related articles

• Amyloid and Tau Explained
• Blood Tests for Alzheimer's
• Brain Imaging: MRI, CT and PET
• Spinal Fluid (CSF) Testing
• Getting a Diagnosis