Preclinical refers to the early phase of a disease -- before signs and symptoms of that disease appear. An accurate diagnosis of the disease is not yet possible, but people who can be identified as having preclinical disease can be targeted for treatments that slow or halt disease progression.
Examples of diseases with a detectable preclinical phase that is fairly long and common include breast cancer, prostate cancer, high blood pressure, and HIV infection.
In Alzheimer's disease, it has become increasingly recognized that amyloid accumulation in the brain occurs decades before symptoms, such as memory loss and personality change, begin. Drugs to slow or halt progression of Alzheimer's disease will likely be most effective in those people with the least amount of brain damage, so identifying people with preclinical Alzheimer's disease will likely be crucial in developing effective treatments.
