Blood p-tau217 has emerged as an accurate biomarker of Alzheimer’s disease (AD) pathology but is also elevated in amyotrophic lateral sclerosis (ALS), likely reflecting confounding contributions from peripheral tau released from denervated muscle fibres. Thus, assays selectively measuring brain-derived (BD) tau and BD-p-tau217 may improve diagnostic specificity. Targeted mass spectrometry after t-tau immunoprecipitation was applied to investigate the presence of CNS tau isoforms (BD-tau without exon 4a) and big tau *isoforms including exon4a) in potential origin tissues (muscle, spinal cord, brain).