A blood test for Alzheimer disease (AD) or other neurodegenerative disorders is highly sought after but has proven elusive. After all, AD pathologic features are confined to the brain, and although a cerebrospinal fluid (CSF) protein signature of AD is plausible, dilution of brain-derived proteins passaging into the bloodstream, cleavage by proteases, binding to abundant proteins in plasma, and clearance by the liver and kidneys mean that blood biomarkers are several steps removed from their origin and much more challenging to detect and interpret. The advent of ultrasensitive assay methods may change this picture. Approaches to quantify peptide or protein biomarkers by refined enzyme-linked immunosorbent assay–like detection methods or by using polymerase chain reaction amplification of a tagged reporter antibody have greatly extended the lower limits of reliable detection.
Douglas Galasko. Searching for Neurodegeneration in the Blood of Patients With Alzheimer Disease. JAMA Neurol. 2017;74(5):510–511. doi:10.1001/jamaneurol.2017.0010