Blood-based biomarkers (BBMs) are blood tests that can detect Alzheimer's disease (AD) brain changes earlier, more cheaply and more easily than traditional methods such as Aβ-PET scans, which are expensive and have limited accessibility. However, blood tests do not always give a clear yes-or-no answer. One of the most promising blood markers, called p-Tau217, can leave a substantial number of people with an uncertain diagnosis. Patients within this "gray zone" may need an expensive brain scan or other testing to determine whether amyloid, a key feature of Alzheimer's disease, is present.