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. Here we analyzed the composition of tau isoforms in muscle tissue after pan-tau-IP including isoforms with exon 4a-long