In 2024, the Alzheimer's Association published changes to the definition of Alzheimer's disease. Those changes included updated criteria to define what Alzheimer's disease is and who has it.

Traditionally, Alzheimer's disease was diagnosed based on symptoms, such as memory loss and changes in thinking and reasoning abilities. If other conditions that cause memory loss were ruled out, such as a stroke, then a diagnosis of probable Alzheimer's disease was made. But Alzheimer's disease could be diagnosed for sure only after a person died. That's when the plaques and tangles in the brain that cause the symptoms of Alzheimer's could be found while looking at the brain tissue with a microscope. Plaques and tangles are made up of proteins that are not typical.

Now the definition of Alzheimer's includes measuring those changes in the brain while a person with Alzheimer's disease is still alive. This is known as a biological definition. It's determined by the presence of what are called biomarkers.

In Alzheimer's disease, a biomarker test may be able to detect the buildup of plaques and tangles in the brain. That buildup can be seen on brain imaging scans. Plaques and tangles also can be detected in samples of blood and samples of the fluid that surrounds the spinal cord and brain, called cerebrospinal fluid.

This updated definition of Alzheimer's allows researchers to more effectively design research clinical trials and identify suitable participants for them. As a result, researchers can learn more about the disease in its earlier stages, even before symptoms start.

And because the brain changes of Alzheimer's start long before symptoms appear, updating the definition to include biological changes may help with earlier diagnosis of the disease overall. In the long term, quicker diagnosis could lead to better management and treatment for Alzheimer's disease.

Oct. 14, 2025

Original article: https://www.mayoclinic.org/diseases-conditions/alzheimers-disease/expert-answers/defining-alzheimers-disease/faq-20436675