Biomarkers of tau pathology. Schematic representation of biomarkers of tau pathology in in the intracellular, membrane, extracellular, cerebrospinal fluid (CSF) and plasma compartments. Color pallet indicates processes that have increased, declined or remained unchanged in Alzheimer's disease (AD). It has been proposed that the leakage of tau into the extracellular and CSF space is secondary to brain damage. Note in AD the increased concentrations of phosphorylated tau in all compartments. p-tau-181, phosphorylated tau protein at the threonine located at position 181; p-tau-231, phosphorylated tau protein at the threonine located at position 231.