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Health A to Z  —  5 minutes

Diagnosing Alzheimer’s disease with a blood test: dream or reality?

January 13th, 2020
Raymond Lepage, PhD, Doctor in Biochemistry
Raymond Lepage, PhD, Doctor in Biochemistry
Science popularizer

A study published on August 1, 2019 in the journal Neurology proposes a new technique based on a simple blood test that would be 94% accurate in diagnosing Alzheimer’s disease in the early stages. The test would be much more sensitive than current imaging techniques, and even enable us to diagnose some cases of Alzheimer’s up to 20 years before the onset of typical symptoms such as memory loss.

Overview of the disease

Alzheimer’s disease is a degenerative brain disease characterized by the build-up of “plaques” and “tangles” in brain tissue. Plaques are dense deposits of an abnormal protein called beta-amyloid. Nerve fibre tangles are associated with another abnormal protein, called the tau protein. These plaques and tangles are responsible for the death of nerve cells and the onset of symptoms typical of the disease.

According to the Alzheimer Society of Canada, the disease will affect more than half a million Canadians in 2019, and this number is expected to double in 15 years. The financial burden of Alzheimer’s currently stands at $10 billion per year in Canada alone.

Diagnosis

Currently, a diagnosis of Alzheimer’s disease is based on tests aimed first at assessing memory loss, decreased functional abilities and changes in behaviour.

Medical imaging

Brain scans using magnetic resonance imaging (MRI), computed tomography (CT-scan) and especially positron emission tomography (PET) round out the diagnosis by revealing the build-up of beta-amyloid plaques. These techniques also help distinguish Alzheimer’s disease from other brain illnesses, including age-related lesions, injuries caused by strokes, tumours, etc.

Blood tests

Blood tests are available, but are only used to rule out other diseases that can cause symptoms similar to Alzheimer’s (e.g. hypothyroidism, severe vitamin B12 deficiency, etc.).

Genetic tests

Some genetic tests, including screening for abnormal versions of the apolipoprotein gene called “apoE4,” can help in assessing the risk of an individual developing Alzheimer’s disease. The presence of a single copy of this abnormal gene is believed to be associated with a two- to five-times higher risk of developing the disease, whereas with two copies of the gene (apoE4/apoE4), the risk could be 10 times higher!

However, genetic tests must be interpreted with care, because many people with two copies of the apoE4 gene will never develop Alzheimer’s disease, while those who have no copies of the gene can still develop it.

Tests can also be done on the APP, PSEN1 and PSEN2 genes in families who have a hereditary form of Alzheimer’s disease. In this case, the disease appears early (before age 65), unlike sporadic Alzheimer’s, which usually occurs after age 65. However, this hereditary form is rather rare, accounting for only about 5% of all cases of Alzheimer’s.

Autopsy

A definitive diagnosis of Alzheimer’s can only be established through an autopsy that includes a microscopic analysis of the brain tissue of people who died from the disease.

New blood test: hope for a definitive diagnosis

We regularly find proposals in scientific publications for blood tests of the same type as the one in the August 1, 2019 issue of Neurology magazine. Why is the new proposal more promising? The reason is that most of the tests previously proposed fail to provide convincing results or are too complex (e.g. cerebrospinal fluid tests) to be used routinely in medicine. The new test is based on a combination of the level of beta-amyloid protein in the blood, the patient’s age and whether or not copies of the apoE4 gene are present. This test is special in that it is relatively simple (i.e. a blood test) and highly accurate (94%) in diagnosing early forms of Alzheimer’s disease. In some cases, the test could even enable us to diagnose the disease before any build-up of beta amyloid plaque is detected through a PET scan.

Treatments

There is currently no treatment available for Alzheimer’s disease, but certain symptoms (e.g. mental function, behavioural changes) may be alleviated, or their progression may be slowed through the use of drugs that regulate the formation of neurotransmitters such as acetylcholine or glutamate. The development of new and more effective drugs is considerably hindered by the near impossibility of establishing a diagnosis early enough to measure the efficacy of these drugs.

Conclusion

What is the point of devising increasingly sensitive tests to diagnose Alzheimer’s disease if no treatment is available? Here lies the great potential of the new test. By diagnosing patients at the earliest stages of the disease, we can verify the real-time and long-term efficacy of treatments vis-à-vis the progression of the disease’s complications. The ability to detect early forms of Alzheimer’s is the first step in the hope of finding a cure for this terrible disease.

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