Can a Blood Test Help Diagnose Alzheimer's Disease?
Alzheimer's disease is linked to two abnormal proteins that build up in the brain: amyloid-beta, which forms plaques, and tau, which forms tangles inside nerve cells. For years, confirming these changes required an amyloid PET scan or a spinal fluid test. Newer blood-based biomarker tests can detect small amounts of related proteins in the bloodstream that reflect these brain changes. For a person who already has memory or thinking problems, a validated test can help a clinician judge whether Alzheimer's disease is likely contributing to the symptoms. A blood test does not replace a complete medical evaluation, including a history, examination, cognitive testing and a search for other causes.
What Do Alzheimer's Blood Tests Measure?
- Phosphorylated tau (p-tau217): a form of the tau protein that rises in the blood when Alzheimer's-related changes are present in the brain. It is currently one of the most studied Alzheimer's blood biomarkers.
- Amyloid-beta 42/40 ratio: compares two forms of amyloid-beta. A lower ratio can be associated with amyloid building up in the brain.
- Combined biomarker testing: some tests combine several measurements, such as p-tau217 with the amyloid-beta 42/40 ratio, to improve how well the test performs.
Not all tests are the same
Commercially available Alzheimer's blood tests differ in what they measure, the cutoffs they use, who they were studied in and how strong their validation evidence is. Some are cleared or validated for use in people with symptoms in specialty settings; others have less supporting data. It is reasonable to ask your clinician which test is being ordered and why.
Who Should Consider an Alzheimer's Blood Test?
Blood biomarker testing is most useful for carefully selected people who are already being evaluated for possible Alzheimer's disease, such as those with:
- Persistent memory problems that family members also notice
- Thinking or memory changes that are getting worse over time
- A diagnosis of mild cognitive impairment (MCI)
- Symptoms that suggest early Alzheimer's dementia
Testing should follow a clinical assessment
The decision to test is best made with a qualified healthcare professional who has examined the person and reviewed their symptoms. Routine screening of healthy adults who have no memory or thinking problems is not recommended.
Who May Not Need Testing?
An Alzheimer's blood test is not automatically right for everyone. It is usually not appropriate for:
- Healthy people with no memory or thinking symptoms
- People who want testing only because a parent or relative had Alzheimer's disease
- People with occasional, ordinary forgetfulness that is not changing or affecting daily life
Family history, genes and blood biomarkers are different
Family history describes who in your family has had dementia. Genetic testing, such as APOE testing, looks at inherited risk. Blood biomarker tests look for signs of Alzheimer's-related changes that may already be present. These are separate questions, and a family history alone does not mean a person needs a blood biomarker test.
How Accurate Are Alzheimer's Blood Tests?
Some well-validated tests have shown high accuracy when used in people with symptoms who are being evaluated in appropriate clinical settings. Even so, families should keep a few points in mind:
- Accuracy varies from test to test and depends on who is being tested and where.
- False-positive and false-negative results can happen.
- Results need to be interpreted alongside symptoms, examination and other test findings.
- An abnormal result does not, by itself, prove that Alzheimer's disease is causing a person's symptoms. Alzheimer's changes can be present alongside other conditions.
What Happens if the Blood Test Is Positive?
A positive or abnormal result usually starts a conversation rather than ending one. Next steps may include discussion of:
- Whether a clinical diagnosis of Alzheimer's disease fits the full picture
- Additional testing, when appropriate
- Whether treatment options might be considered
- A plan for follow-up visits
- Counseling, education and caregiver support
Is more testing always needed?
Some people may need further confirmation, such as an amyloid PET scan or spinal fluid testing, depending on the test used, the clinical situation and the treatment being considered. Others may not. Your clinician can explain what makes sense in your case.
Can Blood Tests Determine Eligibility for Alzheimer's Treatment?
Blood biomarkers are becoming more important in evaluating people with suspected early Alzheimer's disease. Disease-modifying treatments such as lecanemab (Leqembi) and donanemab (Kisunla) require confirmation that amyloid is present. However, a positive blood test does not automatically qualify someone for treatment.
- Eligibility depends on more than a blood result, including the stage of the disease and other medical factors.
- MRI scans are needed before and during treatment for safety monitoring.
- These medicines carry risks, including brain swelling and small areas of bleeding (ARIA).
- Clinicians must follow current prescribing information and applicable testing requirements.
What Should Illinois Families Do if They Notice Memory Problems?
- Write down the changes you have noticed and roughly when they began.
- Schedule an evaluation with a qualified healthcare professional.
- Bring a complete, current list of medications and supplements.
- Consider bringing a family member or trusted friend to the appointment.
- Ask whether formal cognitive testing or other diagnostic studies are appropriate.
- Ask whether an Alzheimer's blood biomarker test would meaningfully help the evaluation.
Look at the whole picture
Memory and thinking problems can have many causes besides Alzheimer's disease, including medication side effects, depression, sleep disorders, thyroid disease and vitamin deficiencies. Some of these are treatable. A comprehensive evaluation is far more useful than relying on any single laboratory result.
Frequently asked questions
- Can a blood test diagnose Alzheimer's disease?
- Some validated blood biomarker tests can provide important evidence of Alzheimer's pathology, but results must be interpreted in the appropriate clinical setting.
- What is p-tau217?
- It is a blood biomarker associated with Alzheimer's-related tau pathology.
- Should everyone over 65 get an Alzheimer's blood test?
- No. Routine blood biomarker screening of cognitively healthy adults is not generally recommended.
- Can an Alzheimer's blood test predict when dementia will develop?
- No. These tests are not reliable tools for predicting when an individual person might develop dementia.
- What if the blood test is negative?
- A negative result may lower the likelihood of Alzheimer's pathology, depending on the test and clinical circumstances, but it does not rule out other causes of memory or thinking problems.
- Does Medicare cover Alzheimer's blood tests?
- Coverage depends on the specific test, the clinical circumstances, current Medicare policies and the date of service. Check with the ordering clinician, the laboratory and your plan before testing.
Finding Alzheimer's and Dementia Support in Illinois
Families often benefit from memory evaluations, caregiver education, community support programs, dementia care planning and local aging and caregiver resources. These pages can help you get started:
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Explore Illinois Dementia Guide for information about dementia symptoms, diagnosis, treatment options, and resources for families and caregivers. Watch additional educational videos as they become available in our dementia video library.
This website provides educational information and does not replace individualized medical evaluation, diagnosis, or treatment. Alzheimer's blood biomarker tests must be interpreted in the context of an appropriate clinical assessment. Consult a qualified healthcare professional about memory concerns and testing decisions. If you believe someone is experiencing a medical emergency, call 911.