Preclinical Alzheimer's disease means that Alzheimer's biomarkersare detected in the brain and blood, but no symptoms of the disease are yet apparent. It's also called "presymptomatic Alzheimer's."
Experts now know that Alzheimer's-related changes in the bloodstream and brain can be detected years before a person begins to show symptoms such as noticeable memory loss or other changes in cognition (thinking skills).
So, "preclinical Alzheimer's" is a new term. In April 2011, a panel of experts recommended new criteria for diagnosing and defining Alzheimer's disease. Whereas the previous criteria (from 1984) required dementia symptoms in order to diagnose Alzheimer's disease, experts now believe that Alzheimer's disease involves a gradual progression through three main stages. These are: a preclinical stage with biomarkers but no symptoms; a middle stage of mild cognitive impairment, where some symptoms are obvious but the person can still function independently; and then a fully symptomatic stage of Alzheimer's dementia.
Note that at this time, biomarkers and other newer tests for preclinical Alzheimer's are mainly used for research purposes. Until an effective treatment for preclinical Alzheimer's is available, doctors won't be routinely trying to diagnose preclinical Alzheimer's disease.