Tau368 improves p-tau diagnostic accuracy for FTLD-tau from FTLD-TDP.

Across tauopathies-Alzheimer's disease (AD), frontotemporal lobar degeneration due to tau (FTLD-tau)-we compared cerebrospinal fluid (CSF) biomarkers of phosphorylated-tau (p-tau) p-tau181, p-tau212, tau368, total tau (t-tau), and the tau368/t-tau ratio, and tested differentiation from non-tau (FTLD-TDP, neuronal α-synuclein disease (αSyn), and controls). In AD, CSF p-tau181 and p-tau212 were significantly higher than in all other groups, including FTLD-tau, while tau368/t-tau was significantly lower. With the aim to combine tau phosphorylation biomarkers (p-tau181 and p-tau212) with biomarkers for severity of tau pathology (tau368), we made ratios between these biomarkers and examined their diagnostic accuracy in FTLD after excluding high/intermediate AD neuropathologic change (ADNC). CSF p-tau181 and p-tau212, as well as p-tau181/tau368 and p-tau212/tau368, were higher in FTLD-tau compared with non-tau groups, and the diagnostic accuracy to discriminate FTLD-tau from FTLD-TDP improved. Levels of the ratios were increased in behavioral and primary progressive aphasia variants of tauopathies when compared to FTLD-TDP. Furthermore, the biomarkers showed significant correlation with both FTLD-tau and AD tau burden at autopsy across brain regions. These results suggest unique patterns of increased relative levels of p-tau epitopes in CSF among ADNC and FTLD-tau could improve the diagnosis of tauopathies and inform inclusion criteria in clinical trial design.