AbstractBackgroundThe neurofilament light chain (NfL) assay is gradually becoming an essential diagnostic tool for the diagnosis of many neurological diseases including amyotrophic lateral sclerosis (ALS). Different methods for the determination of this biomarker in serum have been developed in recent years.MethodsWe measured blood NfL in 429 patients referred to the tertiary ALS center of Montpellier, France using two different ultrasensitive methods (Ella™ and Simoa™) and we compared the clinical performances of these two approaches. We also converted NfL values into age and body mass index‐adjusted Z‐scores to assess cut‐off values of this biomarker in this clinical context.ResultsWe show comparable diagnostic and prognostic performance of Ella™ and Simoa™ technologies in ALS, with specificities and sensitivities exceeding 80% for both. We propose cut‐off values for serum NfL in this clinical context, thus enabling the routine clinical use of this biomarker.ConclusionThe use of NfL in routine clinical practice will help predict survival and improve diagnostic accuracy by distinguishing ALS from other neurological diseases and motor neuron disease mimics.