**3. Cerebrospinal fluid tau**

Enzyme-linked immunosorbent assays (ELISA) has been used extensively for the analysis of tau concentrations in the CSF. The most established immunoassays are developed for the quantification of total-tau (t-tau), which detects non-phosphorylated and phosphorylated tau forms, and phospho-tau (p-tau), which detects tau being phosphorylated at specific epitopes. Among them, the quantification of phospho-threonine-181 (p-tau-181) is broadly extended for research and clinical diagnosis purposes, especially in the context of AD-related pathology. Additionally, several studies have also shown the usefulness of assays detecting phosphoserine-199 (p-tau-199) and phospho-threonine-231 (p-tau-231) [14]. Finally, the recent development of a non-phospho-tau assay detecting non-phosphorylated tau at positions Thr-175, Thr-181 or Thr-231 [15], extended the range of available tools for the dissection of the contribution of each specific tau form in the pathology of the spectrum of neurological conditions with altered tau concentrations in the CSF.
