ST. PAUL, Minin., Feb. 27 (UPI) -- A team of U.S.-based scientists say they have developed a model using a "biological clock" to predict -- with just a simple blood test -- when someone is likely to develop symptoms of Alzheimer's disease. The results of a study published this month in the journal Nature Medicine demonstrate how, by measuring blood plasma levels of a protein in people who are not yet impaired by Alzheimer's symptoms, researchers can predict when cognitive decline will begin