Tau, is found in the microtubul structure of axons and released into the cerebrospinalfluid (CSF) in degenerative central nervous system (CNS) diseases. Therefore, CSF tau titration could be used as a marker in diagnosis of some of the diseases and determining the severity of axonal damage.

One of the basic pathologies of Alzheimer's Disease is intraneural deposits known as neurofibrillary tangles, of which tau protein is the major component. However, tau found in intracellular deposits is hyperphosphorilated, which is different from the one bound to the microtubules, These deposits are thought to interfere with the basic skeleton of the cell, thus, first disturbing axonal transport and flnally leading to cell death. When neuron death overcomes the threshold, cognitive, especially memory deficits are clinically detected.

Total CSF tau and phosphorilated tau levels are markedly elevated in Alzheimer's Disease, but the increase in phosphorilated tau is more specific. Total CSF tau levels could be used to monitor progress of Alzheimer's Disease and in differential diagnosis with other dementias. Increase in tau is also detected in mild cognitive impairment with a potential of conversion to dementia.

CSF tau level could be used as a marker in detecting the level of axonal damage in head trauma, monitoring the effect of neuroprotective agents, determining prognosis and follow-up of clinical recovery . In detecting axonal damage in multiple sclerosis, conflicting results were reported. In acute stroke tau elevation parallel neuronal damage.

As a result, while total CSF tau level could be used as a marker for neuronal damage, phosphorilated tau levels are useful in monitoring formation of neurofibrillary tangles.