NEFL and Brain atrophy: Importantly, serum and CSF NfL levels correlate with disease severity, functional impairment and brain atrophy.70,71 For diagnosis, CSF NfL combined with p-tau181:t-tau ratio provided 80% sensitivity and 81% specificity in differentiating between frontotemporal dementia cases with tau or TDP-43 pathological inclusions, with NfL levels most significantly elevated in those with TDP-43 pathological inclusions.72 This is consistent with C9orf72 gene expansion carriers found to display higher serum NfL than frontotemporal dementia patients without the mutation.