Patient Test Information

Tau_A__42

Also known as:

Alzheimer Biomarkers

Formal name:

Tau Protein and Amyloid Beta 42 Peptide

Related tests:

Phosporylated Tau (P-tau), APOE Genotyping, PSEN1

Why Get Tested?

To help distinguish between Alzheimer disease and other forms of dementia

When to Get Tested?

These tests may be ordered along with cognitive and brain-imaging tests in people who demonstrate some form of dementia. These tests are not routine laboratory tests and are typically available only in research settings or memory clinics.

Sample Required?

A sample of cerebrospinal fluid collected using a spinal tap

Test Preparation Needed?

Your health practitioner will advise you of any preparatory requirements.

How is it used?

Tests for Tau protein and AB42 may be used as supplemental tests to help establish a diagnosis of Alzheimer disease and to distinguish between AD and other forms of dementia. These tests are not widely used or routinely ordered. Use is limited to those suspected of dementia, and testing is typically performed after other causes of a person's symptoms have been ruled out.

When is it ordered?

Tau protein and AB42 tests are primarily performed in research settings and in some memory clinics in conjunction with cognitive tests and brain scans when someone has symptoms of dementia, such as memory loss, behavioral changes, and decreased ability to perform daily life functions. Some health practitioners may order them outside of these settings; however, information on how to interpret the results is limited.

What does the test result mean?

In a symptomatic person, a low AB42 CSF level along with a high tau level reflects an increased likelihood of Alzheimer disease. Recent studies have shown that these abnormal levels may predict a rapid progression of AD. Since these are still in a research phase and not part of routine examinations, it is unclear as to whether all who might have abnormal results would definitely have AD.

Is there anything else I should know?

Assessments of AB42 and tau protein levels do not singularly establish a diagnosis of Alzheimer disease; they represent a common finding that may be used in conjunction with other tests and the person's clinical and family history to suggest a diagnosis of AD.

The clinical use of these tests continues to evolve. For instance, multiple variants of amyloid beta protein, such as AB40 and AB38, have been identified and are being researched for their potential use as AD biomarkers.

If someone has symptoms of dementia, a health practitioner will do a thorough work-up to try to determine the cause. This work-up may include a variety of cognitive tests (such as a Minimal Mental State Exam) to assess memory and possibly PET scanning tests (Pittsburgh Compound-B) of the brain to look for abnormalities.

What is being tested?

These tests measure amyloid beta 42 (AB42) and tau protein in cerebrospinal fluid (CSF). Amyloid beta 42 is a peptide (protein fragment). Increased production of amyloid beta 42 in the brain can lead to the formation of senile plaques. Tau is a structural protein in the brain. When tau becomes saturated with phosphorus groups (P-tau), it can produce neurofibrillary tangles, twisted protein fragments that develop in neurons (nerve cells) and disrupt their ability to transport signals. Neurofibrillary tangles and senile plaques are considered to be the main diagnostic features of Alzheimer disease (AD) that are seen at autopsy.

The measurement of tau and AB42 in CSF is being evaluated for potential roles in the diagnosis and monitoring of AD. It has been shown that a decrease in AB42 with elevated tau or P-tau levels may predict the onset of AD.

How is the sample collected for testing?

A sample of cerebrospinal fluid is collected from the lower back using a lumbar puncture (spinal tap) procedure. This procedure is usually done in a hospital or clinic.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

This is not a blood test but one that is performed on a sample of spinal fluid. Follow any preparatory instructions provided.

  1. How is Alzheimer disease definitively diagnosed?

    Alzheimer disease is currently diagnosed based on cognitive changes and by ruling out other causes of these changes. It is definitively confirmed after death by looking for microscopic changes in a person's brain tissue. The microscopic evaluation involves looking for the number of senile plaques and neurofibrillary tangles found in the brain. Characteristic changes on brain scans (MRI or PET scans) and/or low AB42 and high tau protein levels in CSF (where available) may be ordered to help in the diagnosis.

  2. Can I have my blood tested for Aβ42 and tau instead of my CSF?

    Not at this time. Studies of blood measurements of AB42 and tau have not shown them to be useful reflections of what is occurring in a person's brain.