Can a Blood Test Diagnose Alzheimer’s Disease?

In our previous post, we discussed the difference between normal aging, mild cognitive impairment, and dementia—and what to expect when you bring concerns about memory or thinking to your primary care physician. We also emphasized an important point: not every memory problem is caused by Alzheimer’s disease.

Until recently, determining whether Alzheimer’s was contributing to cognitive decline often required specialized testing, such as a PET scan or analysis of cerebrospinal fluid obtained through a lumbar puncture. These tests remain important, but they may be expensive, invasive, difficult to access, or limited to specialty centers.

New blood tests may help change that.

These tests represent an exciting advance, but they are not simple “yes or no” tests for dementia. Understanding what they can—and cannot—tell us is important before deciding whether testing is appropriate.

What Do Alzheimer’s Blood Tests Measure?

Alzheimer’s disease is associated with the abnormal buildup of two proteins in the brain: beta-amyloid and tau. Beta-amyloid collects between brain cells and forms plaques. Tau accumulates inside brain cells and forms tangles. These changes interfere with communication between brain cells and may begin years before someone develops obvious symptoms.

Newer blood tests measure proteins associated with these changes. One of the most promising is called p-tau217, a form of the tau protein that can help estimate whether Alzheimer’s-related amyloid plaques are likely to be present in the brain.

Some tests also measure beta-amyloid or combine amyloid and tau measurements to improve accuracy.

In 2025, the FDA cleared the first blood test intended to help identify amyloid plaques associated with Alzheimer’s disease in adults age 55 and older who are already experiencing signs or symptoms of cognitive decline. Additional Alzheimer’s-related blood tests are also becoming available through commercial laboratories. However, the tests are not all identical. They may measure different proteins, use different laboratory methods, and have different levels of evidence supporting their accuracy.

Can a Blood Test Diagnose Alzheimer’s Disease?

Not by itself.

A blood test may help determine whether the biological changes associated with Alzheimer’s disease are likely to be present. That information can be extremely useful, but it does not explain the entire clinical picture.

A positive test does not automatically mean:

• Alzheimer’s is the only cause of the person’s symptoms

• The person currently has dementia

• The person will decline at a particular rate

• Every memory problem is explained by the result

Likewise, a negative result may make Alzheimer’s disease less likely, but it does not tell us what is causing the symptoms. Memory and thinking problems may also be related to:

• Medication side effects

• Depression or anxiety

• Poor sleep or sleep apnea

• Thyroid disease

• Vitamin deficiencies

• Alcohol use

• Hearing loss

• Previous strokes or vascular disease

• Other neurological disorders

It is also possible for more than one condition to contribute to cognitive decline. For example, someone may have both Alzheimer’s-related changes and vascular damage from previous strokes or long-standing high blood pressure.

A blood biomarker may identify evidence of Alzheimer’s-related changes, but the diagnosis still depends on the person’s symptoms, cognitive function, medical history, examination, and daily abilities.

Who Might Benefit From Testing?

An Alzheimer’s blood test may be helpful for someone who:

• Has ongoing concerns about memory or thinking

• Has measurable cognitive impairment

• Has symptoms that could reasonably be caused by Alzheimer’s disease

• Has already had an appropriate medical evaluation

• Understands what the result may—and may not—show

• Is willing to pursue further evaluation if the result is positive or unclear

These tests are currently intended primarily for people who are already experiencing symptoms. They are not recommended as routine screening tests for people with normal memory and thinking abilities.

Ordering a test “just to know” may create more uncertainty than clarity. Alzheimer’s-related changes can sometimes be detected before symptoms develop, but a test may not tell us whether or when that person will experience cognitive decline.

What Should Happen Before the Test Is Ordered?

A blood biomarker test should usually be part of a broader evaluation—not the first and only step.

Your primary care physician may:

• Ask when the symptoms began and whether they are worsening

• Discuss specific examples with the patient and family

• Review medications and supplements

• Evaluate mood, sleep, hearing, and alcohol use

• Perform an office-based cognitive assessment

• Order routine laboratory testing for reversible or contributing causes

• Consider an MRI or CT scan when appropriate

• Assess whether memory changes are affecting medications, finances, driving, cooking, or other daily activities

This evaluation helps determine whether an Alzheimer’s blood test is likely to provide useful information.

What Do the Results Mean?

The way results are reported varies by test, but they may be classified as positive, negative, intermediate, or as an estimated likelihood that amyloid plaques are present.

A Negative Result

A negative result from a reliable test may make Alzheimer’s-related amyloid changes much less likely. This can help your physician focus on other possible explanations for the symptoms. It does not mean the symptoms are imaginary, and it does not rule out every other cause of cognitive decline.

A Positive Result

A positive result suggests that Alzheimer’s-related changes may be present. Depending on the specific test and clinical circumstances, your physician may recommend evaluation by a neurologist or memory specialist. Some patients may also need confirmation with an amyloid PET scan or cerebrospinal fluid testing, particularly if the result will be used to make an important treatment decision.

An Intermediate or Unclear Result

Sometimes the result falls into an uncertain range. This does not confirm or exclude Alzheimer’s disease. It means the blood test alone cannot answer the question, and additional evaluation may be needed.

False-positive and false-negative results are also possible. This is why these tests should not be interpreted without considering the rest of the clinical picture.

Is This the Same as Genetic Testing?

No. Alzheimer’s blood biomarker tests measure proteins associated with changes occurring in the brain. Genetic tests look for inherited variations that may affect a person’s risk. The best-known Alzheimer’s risk gene is APOE. One form, called APOE-e4, increases the likelihood of developing Alzheimer’s disease, but it does not determine whether someone will develop it. Many people who carry APOE-e4 never develop Alzheimer’s, while many people with Alzheimer’s do not carry it. For that reason, APOE testing is not routinely recommended simply to predict whether a healthy person will eventually develop dementia.

Rare genetic variants can directly cause inherited, early-onset Alzheimer’s disease, but these account for a very small percentage of cases. They are generally considered when several close relatives have developed Alzheimer’s at unusually young ages. Testing in this situation should typically involve a neurologist or genetic counselor.

APOE testing may also be discussed when someone is being evaluated for certain newer Alzheimer’s treatments because the result may affect the risk of treatment-related side effects. That is different from using genetic testing to diagnose the cause of memory symptoms.

A risk gene is not a diagnosis, and an Alzheimer’s biomarker test is not a genetic test.

Why Could an Earlier and More Accurate Diagnosis Matter?

Identifying the cause of cognitive decline can help patients and families:

• Better understand what is happening

• Address reversible or contributing conditions

• Avoid unnecessary or ineffective treatments

• Make plans regarding medications, driving, finances, and future care

• Access support services and clinical trials

• Discuss whether an Alzheimer’s-specific treatment may be appropriate

Newer medications that target amyloid are available for carefully selected patients with mild cognitive impairment or mild dementia caused by Alzheimer’s disease. Before one of these treatments can be considered, the presence of amyloid must be confirmed and additional safety testing is required.

A positive blood test does not automatically mean that someone qualifies for treatment. These decisions typically require specialist involvement and a careful discussion of the potential benefits, risks, and monitoring requirements.

What Is the Role of Your Primary Care Physician?

Primary care remains the best place to begin when memory or thinking changes are first noticed.

Your primary care physician can look for reversible causes, perform an initial cognitive assessment, evaluate how symptoms are affecting daily life, and determine whether laboratory testing, brain imaging, or referral to a specialist is appropriate. In some cases, an Alzheimer’s blood test may help clarify the next step. In others, a different part of the evaluation may be more useful.

The goal is not simply to order the newest test. The goal is to determine what is changing, understand why it may be happening, and decide what information will meaningfully improve the patient’s care.

Questions to Ask Before Having an Alzheimer’s Blood Test

Before testing, consider asking:

• What specific test are you ordering?

• What does it measure?

• Is it FDA cleared?

• Is it appropriate for someone with my symptoms?

• What would a positive or negative result mean?

• Could the result be unclear?

• Would I need additional testing afterward?

• Will the result change my treatment or care plan?

• Is the test covered by insurance?

Final Thoughts

Blood tests for Alzheimer’s disease are an important and promising advance. They may make it easier to identify—or rule out—Alzheimer’s-related brain changes without immediately requiring a PET scan or lumbar puncture. But they are not stand-alone tests for dementia. They are most useful when ordered for the right patient, for the right reason, and interpreted as one part of a complete evaluation. If you or someone you love is experiencing persistent changes in memory, judgment, language, or daily function, your primary care physician remains the best place to begin.

In our next post, we’ll discuss what happens after a diagnosis of mild cognitive impairment or dementia—including treatment options, lifestyle interventions, safety planning, and practical steps families can take early.

---

At Cardona Direct Primary Care, Dr. Cardona and Dr. Garland provide personalized healthcare, including direct primary care, obesity medicine, and aesthetic treatments. Dr. Cardona is board-certified in both family medicine and obesity medicine and has a special interest in medical weight loss. Dr. Garland is board-certified in family medicine and is a certified medical marijuana provider. She has a special interest in skincare and is eager to assist with your individual skincare needs. Located in Jacksonville, FL, we are dedicated to prioritizing you. Contact us at 904-551-4625 or visit our website at www.cardonadpc.com to learn more and schedule your appointment today!