Overview
What Are the Blood Tests for Alzheimer’s Disease?
Blood tests can help doctors look for signs of Alzheimer disease in the body. These signs are called markers. The most common markers are the amyloid-beta 42/40 ratio, phosphorylated tau 217 (p-tau217), and phosphorylated tau 181 (p-tau181). A lower amyloid-beta 42/40 ratio may suggest amyloid buildup in the brain. Higher p-tau217 or p-tau181 may suggest changes linked to Alzheimer disease. Of these markers, p-tau217 appears to be the most accurate overall.These tests can show Alzheimer-related changes, but they do not diagnose dementia by themselves. A diagnosis of dementia requires memory or thinking symptoms that meet clinical criteria and affect a person’s ability to manage daily life independently.
Biological changes on a test do not automatically mean a person has dementia.
Who Should Get Tested?
These blood tests are best used for people who already have memory or thinking problems. They should be ordered as part of a full neurological evaluation. They are not recommended for people with no symptoms, even if Alzheimer disease runs in the family. In people without memory problems, a positive test is more likely to be misleading. A blood test result should always be reviewed along with the person’s symptoms, memory testing, exam, lab work, and brain imaging.What to Expect
During the Test
The actual test is a simple lab draw. It requires no special preparation or recovery.
Test Interpretation
Several years ago testing was done with cerebral spinal fluid obtained via a lumbar puncture (also called a spinal tap). Blood tests are much easier for the patients, but the lab interpretation can be more challenging. Spinal fluid testing is more established because the fluid is close to the brain and spinal cord. Blood testing is harder to interpret because these proteins are found in very small amounts in the blood.Results can also be affected by health problems outside the brain. Different labs may use different test methods and different rules for what counts as normal or abnormal. Because of this, tests with similar names may not give the same results. Some well-studied blood tests now work very well in the right patients. Other tests have less evidence behind them.
Your physician will explain the exact tests being performed and what the results can mean.
False Positive Tests
A false positive means the test is abnormal (meaning it indicates the Alzheimer’s proteins are present in the blood) even though the person may not have Alzheimer disease causing symptoms. This is more likely when the test is used in people with no memory problems.In one study, about 1 in 5 positive blood tests (20%) in people without memory symptoms was not confirmed by further specialized testing. This is why these tests should not be used as general screening tests or reassurance tests. Kidney disease and some other medical conditions may also affect results. Even certain supplements can affect these blood tests.
This is why the results are only part of the overall diagnostic tests and exam. Your physician will use the results and other information, such as cognitive function tests, imaging and a physical exam, to determine the most likely cause of memory issues.
Alzheimer disease can occur with other dementias
An abnormal Alzheimer blood test can also occur in someone who has another type of dementia, such as Lewy body dementia, Parkinson disease dementia, frontotemporal dementia, or vascular dementia. This does not always mean the first diagnosis was wrong. Older adults can have more than one brain disease at the same time, known as mixed dementia.Alzheimer changes can occur along with Lewy body disease, blood vessel disease, or other brain changes. Amyloid plaques can also be seen in some older adults who do not have memory problems. A blood test may show Alzheimer-related changes, but it cannot show how much those changes are causing the person’s symptoms. The doctor still needs to consider the symptoms, exam, and brain imaging to find the most likely cause of memory decline.
Next Steps
Be Aware This Is A Dynamic Field
This field is changing quickly. Many Alzheimer blood tests are now available. Some are ordered by clinicians, and others are advertised directly to patients. Some tests have been reviewed by the FDA for certain uses, and others have not. FDA-clearance does not mean the test can diagnose Alzheimer disease by itself. It also does not mean the test should be used to screen people with no memory symptoms.Blood tests are promising, but doctors still need better information about how to handle results that are not clearly normal or abnormal, medical conditions that may affect results, and how well these tests work in different groups of people. Recommendations will continue to change as more evidence becomes available.