Can the NfL test rule out the diagnosis of ALS?

A diagnosis of ALS cannot be made by measuring neurofilament light chain alone. NfL is therefore not a diagnostic marker, but a biomarker for ALS progression. Thus, slow ALS (with little progression) can also progress without an increase in NfL. A normal (age-adjusted) NfL value therefore does not rule out the diagnosis of ALS (limited “sensitivity” of NfL). Conversely, an elevated NfL value does not rule out another neurological disease, as several neurological diagnoses are known (e.g. a severe form of polyneuropathy), which may also show an NfL elevation (limited “specificity” of NfL).