Anyone who has ever visited a doctor’s office is familiar with the use of blood tests for the diagnosis of various diseases. Because blood comes in contact with all organs of the body, it carries markers of the health of these organs. It is an easily accessible body fluid, can be drawn repeatedly to follow the progress of a disease and, in most cases, blood tests are relatively inexpensive.
It is therefore not surprising news that a possible blood test for dementia, in particular Alzheimer’s disease, gets much public attention.
Diagnostic testing for Alzheimer’s has recently made significant progress. While an MRI brain scan will show some abnormality in Alzheimer’s, this is non-specific and appears somewhat late in the course of the disease.
A positron emission tomographic (PET) scan may be done to get a picture of amyloid deposits, a protein that accumulates in the brain with Alzheimer’s, and a spinal fluid tap can detect abnormal levels of beta-amyloid (Aβ) and tau proteins found in the brain of Alzheimer’s patients.
However, both tests are expensive and not readily available. The spinal tap is also much more uncomfortable than obtaining blood from a vein. These tests are not subsidised by the government, partly because they do not influence the treatment patients will receive since there are no current drugs that will cure or even modify the course of Alzheimer’s.
Many patients, nevertheless, want a specific diagnosis, which a clinical assessment can only guess at. As the promise of early intervention comes nearer, the call for a blood test is getting louder.
What Blood Can Tell Us
Blood has some disadvantages compared to spinal fluid. It is separated from the brain by what is called the “blood brain barrier”, which allows only some molecules across. While the spinal fluid has a more specific association with the brain, the blood has signals from all organs, so it may be difficult to establish that a particular signal detected in peripheral blood is actually coming from the brain.
Despite these limitations, considerable progress has been made in recent years, although the knowledge is still a number of years away from the clinic.