NEFL and delirium: Serum NfL levels increase during aging and are elevated in various neurologic conditions, including traumatic brain injury, multiple sclerosis, and neurodegenerative diseases.6, 7, 8, 9, 10, 11 In cognitively normal older adults undergoing elective surgery, peak delirium severity on the second post-operative day was associated with higher Nfl concentrations in both serum and CSF compared with patients who had no delirium.12