NEFL and Alzheimer disease: One promising neurodegeneration biomarker in CSF is neurofilament light (NfL), which increases in a number of neurologic conditions, including AD.4, –, 8 NfL can be detected in serum using standard immunoassay formats,9,10 but many samples have concentrations below the analytical sensitivity of the methods.11 We therefore used a recently developed immunoassay based on the single molecule array (Simoa)12 that is 25-fold as sensitive as the previous electrochemiluminescence-based method for NfL.11