Last month, The US Food and Drug Administration approved a new blood test for assisting the diagnosis of Alzheimer’s disease. Produced by Roche, Elecsys pTau181 measures the concentration of a specific molecule—a phosphorylated form of the tau protein—in the blood. Elecsys pTau181 looks in the blood plasma for a form of the tau protein that has a phosphate group attached, which is often found in elevated amounts in Alzheimer’s patients. Subjecting all elderly people with suspected symptoms of cognitive decline to PET scans and cerebrospinal fluid sampling is impractical, so this is where blood testing for Alzheimer’s comes in. In settings where the overall prevalence of amyloid disease is low, a negative result from this test is 97.9 percent reliable.