Blood tests for Alzheimer's disease

What are these blood tests looking for?

Alzheimer's disease is associated with changes in two proteins in the brain: amyloid and tau. Several blood tests look for traces of those changes. Other markers can show that brain cells or supporting cells are under stress without identifying one particular disease.

This distinction matters because the markers do not all answer the same question.

p-tau217

Tau is a protein found in nerve cells. In Alzheimer's disease, tau undergoes several changes. One of them can be measured as phosphorylated tau 217, or p-tau217, in the blood.

Among currently studied blood markers, p-tau217 has been one of the strongest signals that Alzheimer's-related amyloid and tau biology is present. It does not count plaques or tangles directly. It measures a change in tau that is closely associated with that biology.

The number 217 identifies the location on the tau protein being measured. It is not a score or a stage of disease.

p-tau181

p-tau181 measures a different phosphorylated location on the same tau protein. It has been studied for longer and can also provide evidence of Alzheimer's-related biology.

In many recent head-to-head studies, p-tau217 has separated Alzheimer's pathology from other conditions more accurately than p-tau181. That does not make every p-tau217 test automatically better. The laboratory method, the thresholds, and the population in which a test was validated all matter.

Amyloid beta 42/40

Amyloid beta occurs in several forms. Two common forms are called A beta 42 and A beta 40. When amyloid plaques accumulate in the brain, the amount of A beta 42 relative to A beta 40 often changes in blood and spinal fluid.

The A beta 42/40 ratio compares the two forms. Using a ratio helps account for the fact that people naturally produce different overall amounts of amyloid beta. A lower ratio can provide evidence that amyloid plaque is more likely to be present in the brain.

The change in blood can be small, so collection, handling, and the testing platform matter. A result from one assay should not be compared directly with the reference range or numerical value from another.

NfL and GFAP

Neurofilament light, usually shortened to NfL, is released when nerve cell fibers are injured. An elevated level can indicate active neurological injury, but it does not identify the cause. NfL can rise in Alzheimer's disease, frontotemporal degeneration, motor neuron disease, stroke, and other conditions.

Glial fibrillary acidic protein, or GFAP, is associated with the response of astrocytes, cells that support and protect nerve cells. GFAP can rise with Alzheimer's-related amyloid changes, but it can also change in other forms of brain injury or inflammation.

NfL and GFAP may add useful context. Neither is a stand-alone test for Alzheimer's disease.

What does a result mean?

The report may use categories such as positive, negative, or intermediate. Those words apply to the specific assay that was ordered.

A positive result means the test found a pattern associated with a higher likelihood of Alzheimer's pathology. The next question is whether that biology fits the person's symptoms and whether another process may also be present.

A negative result makes Alzheimer's pathology less likely according to that test. It does not explain what is causing the symptoms. If the clinical picture and the blood result disagree, additional testing may still be useful.

An intermediate result means the value falls between the test's positive and negative thresholds. It is not a failed test. It means the blood result is not decisive enough to answer the question by itself.

Some tests are accurate enough to confirm or exclude amyloid pathology in a specialty setting. Others are better used to decide who may benefit from an amyloid PET scan or spinal fluid testing. The name of the marker alone does not tell you which kind of test was used.

Why can two people with the same result have different symptoms?

A biomarker measures one part of a larger story. It does not show which brain networks are most affected, how much function has changed, or how resilient a particular person may be.

Alzheimer's pathology can also occur alongside vascular injury, Lewy body disease, TDP-43-related disease, or another process. A positive Alzheimer's biomarker can therefore be both accurate and incomplete. It may identify one real contributor without explaining every symptom.

Age, kidney function, cardiovascular and metabolic illness, and other health factors may also affect some blood-marker levels. This is one reason the result belongs inside a clinical evaluation rather than being interpreted from the patient portal alone.

When might a clinician order one?

Current clinical guidance is focused on people who have objective cognitive impairment and are being evaluated in specialty care. A blood biomarker may help when knowing whether Alzheimer's biology is present could:

• clarify the likely cause of symptoms;
• determine whether PET or spinal fluid testing is still needed;
• support a treatment decision that requires evidence of amyloid pathology;
• explain why the observed symptoms and other test results do or do not fit together.

These tests are not designed to answer the general question, "Will I develop Alzheimer's disease someday?" in a person without symptoms. Risk assessment, diagnosis, and treatment eligibility are different questions.

The useful way to read the result

A blood biomarker can make Alzheimer's disease more likely or less likely. It becomes clinically meaningful when it is connected to the person's symptoms, daily function, examination, imaging, and goals.

The most useful question is not only, "Is the test positive?" It is, "What does this result explain, what does it leave unanswered, and what should we do next?"