The secret is the levels of a protein in a person’s blood. Measuring the changes in levels of a certain portion in a patient’s blood can help show damage that’s happening to the brain long before a patient starts to show symptoms, The Guardian reports.

There’s currently no way to stop the progression of Alzheimer’s or slow its progression although people are working on a cure. The hope is that the blood test would enable doctors to have a better idea of when a patient might start showing symptoms of the disease. It could also be useful in determining whether an Alzheimer’s medication shows promise.

The research was done at the German Center for Neurodegenerative Diseases in Tübingen and published in the journal Nature Medicine.

While the study is promising, it’s not a fool-proof test. The study looked at people who were genetically predisposed to the disease, which only affects 1% of the world’s population, which is what allowed it to determine when the portion changes started to happen, simply because they had an idea on when the disease was likely to start presenting symptoms in those patients.

Patients can also see an increase in the same portion after a traumatic brain injury, and if they have multiple sclerosis.