Understanding Lab Tests for Alzheimer's: What You Need to Know
- Dr. Koren
- Aug 20
- 8 min read
It's natural to feel concerned about Alzheimer's disease, especially if you have a family history or notice changes in your own memory or thinking. Modern laboratory diagnostics provide tools for assessing both hereditary and non-hereditary risks and detecting early biological brain changes before symptoms appear. Although no single test can diagnose Alzheimer's with complete certainty, the test results may offer crucial insights into risk and aid in diagnosis.
This post breaks down the types of lab tests currently available for Alzheimer's, keeping it straightforward and easy to understand.
Part 1: Evaluating Your Risk for Alzheimer's
Risk assessment can be divided into hereditary (genetic) and non-hereditary factors.
A. Hereditary (Genetic) Risk Testing
Alzheimer’s can run in families, but the genetics are complex.
1. Early-Onset Alzheimer’s (Rare, usually before age 65)
Caused by mutations in one of three genes:
APP (amyloid precursor protein)
PSEN1 (presenilin 1)
PSEN2 (presenilin 2)
These mutations are deterministic—if you inherit one, Alzheimer’s is almost inevitable, often in mid-life. People with Down syndrome are at much higher risk of developing early-onset Alzheimer’s disease compared to the general population. Down syndrome is caused by having an extra copy of chromosome 21 (trisomy 21). On chromosome 21 sits the APP gene (amyloid precursor protein). With three copies of this gene instead of the usual two, people with Down Syndrome make extra amyloid-beta protein. Amyloid-beta is the substance that clumps into plaques in the brain—a hallmark of Alzheimer’s.
Who might consider this: Individuals with a strong family history of early-onset dementia, where multiple family members have been diagnosed in their 40s or 50s.
Important considerations: Genetic testing has significant implications. It's crucial to discuss the pros, cons, and potential emotional impact with a genetic counselor before undergoing such testing. A positive result doesn't mean you will get Alzheimer's if the mutation isn't in one of the causative genes, and a negative result doesn't eliminate the risk of late-onset Alzheimer's.
2. Late-Onset Alzheimer’s (Most common, after age 65)
No single “Alzheimer’s gene,” but certain variants influence risk.
The "Risk Factor" Gene, Apolipoprotein E (APOE) is the most well-known genetic risk factor for the much more common, late-onset form of Alzheimer's (symptoms typically after age 65).
The APOE gene comes in three common versions, or alleles. We inherit one copy from each parent.
What the test looks for: Which two versions of the APOE gene you carry.
APOEϵ4: The "risk" allele. Having one copy of APOEϵ4 increases your risk by approximately 3 times. Having two copies increases the risk by about 8-12 times.
APOEϵ3: The most common and "neutral" allele. It doesn't appear to change risk.
APOEϵ2: A rare, "protective" allele that may slightly reduce the risk of developing Alzheimer's.
What a result means: This is crucial—APOEϵ4 is a risk factor, not a destiny. Many people with the ϵ4 allele never develop Alzheimer's, and many people who get Alzheimer's do not have the ϵ4 allele. For this reason, this test is generally not recommended as a predictive test for the general population. However, it can be useful information in a full diagnostic workup if someone is already experiencing symptoms.
B. Non-Hereditary Risk Biomarkers
Most Alzheimer’s cases are due to a mix of genetic, lifestyle and environmental factors. Blood work can identify modifiable risks that also contribute to cognitive decline:
Blood sugar (A1C, fasting glucose) – diabetes increases Alzheimer’s risk.
Cholesterol and triglycerides - not directly linked to Alzheimer's dementia but important for overall health of blood vessels and brain health.
Vitamin deficiencies – especially folic acid, B12
While these tests don’t diagnose Alzheimer’s, they highlight health issues that may worsen cognitive decline.
Part 2: Lab Tests for Diagnosing Alzheimer's Disease
When doctors suspect Alzheimer’s, the gold standard is a clinical evaluation—history, physical exam, neurocognitive testing—combined with lab and imaging tests to rule out other causes (like thyroid problems, depression, medication effects). But now, specific biomarker tests can directly detect Alzheimer’s-related brain changes.
A. Cerebrospinal Fluid (CSF) Biomarkers
What the test looks for:
Amyloid-beta 42 (Aβ42): In Alzheimer's, this protein clumps together to form amyloid plaques in the brain. As a result, less of it is free-floating in the CSF. A low level of Aβ42 in the CSF suggests amyloid plaques are present in the brain. Often, doctors look at the ratio of Aβ42 to another form, Aβ40, for even greater accuracy.
Phosphorylated Tau (p-Tau): Tau is a protein that stabilizes nerve cells. In Alzheimer's, it becomes abnormally modified (phosphorylated) and forms neurofibrillary tangles inside neurons. High levels of p-Tau in the CSF are a very specific indicator of Alzheimer's-related tangles.
Total Tau (t-Tau): This measures the overall amount of Tau protein released from damaged neurons. High levels indicate neurodegeneration but are not specific to Alzheimer's (it can be elevated in stroke or other dementias).
When a person's CSF shows the classic pattern—low Aβ42 and high p-Tau—it provides strong biological evidence supporting a diagnosis of Alzheimer's disease.
CSF testing is accurate but invasive, requires a lumbar puncture (“spinal tap”), so it’s not always the first choice.
B. Blood-Based Biomarkers (New & Growing Field)
Recent advances mean some Alzheimer’s markers can now be detected in a simple blood draw—more accessible and less invasive.
What the test looks for: Ultrasensitive technology can now detect the same Alzheimer's pathologies in the blood that were once only visible in CSF or on expensive PET scans.
Plasma p-Tau (p-tau217 or p-tau181): This is the star player. The level of specific forms of phosphorylated Tau in the blood has shown remarkable accuracy in identifying both amyloid plaques and tau tangles in the brain. It correlates strongly with results from both CSF tests and PET scans.
Plasma Amyloid Beta Ratio (Aβ42/Aβ40): Similar to the CSF test, this blood test measures the ratio of the two amyloid forms to detect evidence of brain plaque accumulation.
Neurofilament Light Chain (NfL): Like t-Tau in the CSF, NfL in the blood is a general marker of nerve damage and is not specific to Alzheimer's, but can help track the progression of neurodegeneration.
The currently available laboratory tests include:
1. PrecivityAD™ Test (C₂N Diagnostics)
Only your healthcare provider can order the PrecivityAD® blood test for you.
The test measures specific amyloid beta (Aβ42/40 ratio) and apolipoprotein E (ApoE) peptides in the blood
The main result of the PrecivityAD® blood test is the Amyloid Probability Score (APS), which is a numeric score ranging from zero to 100 that is calculated using a proprietary algorithm based on the patient’s Aβ42/40 ratio, ApoE and age.
The PrecivityAD® blood test report includes the APS with interpretation and a results table. A high score means amyloid plaques are likely in the brain. A low score means amyloid is unlikely. A middle/indeterminate score means the test couldn’t clearly say either way and signals when follow-up with a PET scan or spinal fluid test may be needed.
2. PrecivityAD2™ Test (C₂N Diagnostics)
Only your healthcare provider can order the PrecivityAD2® blood test for you.
The test measures specific amyloid beta (Aβ42/40 ratio) and tau peptide (p-tau217/np-tau217 ratio) concentrations in the blood
The main result of the PrecivityAD2™ blood test is the Amyloid Probability Score 2 (APS2), which is a numeric score ranging from zero to 100 that is calculated using a proprietary algorithm based on the patient’s Aβ42/40 Ratio and p-tau217 Ratio. The PrecivityAD2™ blood test report includes the APS2 with interpretation and a results table.
The PrecivityAD2™ and the PrecivityAD® blood tests are intended for use in patients aged 55 and older with signs or symptoms of mild cognitive impairment or dementia who are undergoing evaluation of Alzheimer’s disease or dementia.
The PrecivityAD® and PrecivityAD2™ blood tests are NOT intended for:
Use in patients less than 55 years old at time of blood collection
Use as a screening test
Use in individuals without symptoms
Use as a serial test for assessment of longitudinal changes
3. AD-Detect™ (Quest Diagnostics)
Measures the plasma Aβ42/Aβ40 ratio using a blood sample
The test gives a numeric ratio, with a cut-off for what is “likely positive” or “likely negative.” A low ratio means amyloid plaques may be present. A higher ratio means they are less likely.
Studies and analyses indicate that many individuals without brain amyloid pathology may receive a false positive (abnormal) result, leading some dementia specialists to express significant concern, labeling the test "low-quality" and warning that its widespread use could lead to an "absolute catastrophe" of patient anxiety, unnecessary and expensive follow-up procedures (like PET scans).
No blood test is 100% reliable—results must be interpreted in the bigger picture of symptoms and other tests. A negative test doesn’t absolutely rule out Alzheimer’s, and a positive test doesn’t guarantee it either. Think of these tests as important clues, not final answers.
C. Imaging with Laboratory Correlation
While not “lab tests” in the traditional sense, certain brain scans are often interpreted alongside biomarker results:
Amyloid PET scans – show plaque deposits.
Tau PET scans – show tangle deposits.
The use of these powerful but costly imaging techniques is not appropriate for every patient. They are more likely to be used for the evaluation of persistent or progressive symptoms when diagnosis remains uncertain or to determine eligibility for specific amyloid-targeting therapies.
Putting It All Together: What This Means for You
The journey to an Alzheimer's diagnosis is no longer a path of uncertainty. It is a comprehensive process that combines:
Clinical Evaluation: Any individual concerned about their memory or risk for Alzheimer's disease should begin with a conversation with their primary care physician. However, for the interpretation of advanced biomarker tests (CSF, blood, or PET), it is crucial to consult with a dementia specialist, such as a neurologist or geriatrician, who has the expertise to integrate these results into a comprehensive clinical picture.
Cognitive Testing: Standardized tests to objectively measure memory and thinking skills.
Biomarker Testing: A CSF or blood test to confirm the presence of the disease's underlying biology. Not all commercial biomarker tests are created equal. Decisions should be guided by tests that have been rigorously validated and have demonstrated high accuracy when compared to established gold standards like PET or CSF analysis.
Brain Imaging: An MRI to rule out other causes like tumors or strokes, and PET scans (if needed) to visualize amyloid or tau in the brain.
Genetic Counseling: For any form of genetic testing, whether for the deterministic early-onset genes (APP, PSEN1, PSEN2) or the late-onset risk factor (APOE), consultation with a certified genetic counselor is essential. A counselor can provide the necessary education on the profound personal, familial, ethical, and psychosocial implications of learning one's genetic status before any testing is performed.
If you are worried about your memory or your risk for Alzheimer's, the most important step is to start a conversation with your doctor. They can guide you through this process, helping you understand which tests, if any, are right for you. We are at a pivotal moment where seeing the disease clearly allows us to fight it more effectively, a prospect that brings renewed hope to millions of families worldwide.
If you'd like to discuss your personal situation and receive individualized advice, schedule an appointment with the Institute for Diabetes, Endocrinology, Adiposity, and Longevity today.
Till next time,
Dr. Koren
DISCLAIMER: The content on this webpage is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your doctor or qualified healthcare provider. Never disregard professional medical advice or delay in seeking it because of something you have read or watched on this website. The mention of any product, treatment, or organization on this website does not indicate the author's endorsement. The product names referenced here are trademarks owned by their respective manufacturers. The author disclaims any legal liability for personal injury or any other damage or loss resulting directly or indirectly from the use or misuse of this website's contents.
Comments