Tau isoform mapping across tissues and biofluids reveals a signature distinguishing Alzheimer's disease from amyotrophic lateral sclerosis.

Blood phosphorylated tau 217 (p-tau217) accurately detects Alzheimer's disease (AD) pathology but is also elevated in amyotrophic lateral sclerosis (ALS), likely reflecting peripheral (big) tau released from muscle. Assays selectively measuring brain-derived (BD-) tau and BD-p-tau217 are therefore needed. We analysed serum BD-tau and p-tau217 in ALS (n = 139), AD (n = 101) and controls (n = 75) using direct immunoassays and after BD-Tau immunoprecipitation (BD-Tau-IP), and tau species in brain (n = 16), spinal cord (n = 11) and muscle (n = 10) by targeted mass spectrometry. With the direct immunoassays, BD-tau and p-tau217 were increased in AD and ALS versus controls. After BD-Tau-IP, BD-tau and BD-p-tau217 were elevated in AD only, improving diagnostic accuracy. Mass spectrometry revealed a central-to-peripheral shift from BD-tau to big tau predominance, with exclusively big p-tau217 detected in muscle. Selective detection of BD-tau species improves the specificity of blood AD biomarkers by reducing peripheral tau confounding.