Clinical indications for analysis of Alzheimer's disease CSF biomarkers

The cerebrospinal fluid (CSF) biomarkers β-amyloid_1-42 (Aβ_1-42), total tau protein (T-tau) and hyperphosphorylated tau (P-tau_181P) are well-validated and are increasingly used in clinical practice as an affirmative diagnostic tool for Alzheimer's disease (AD). These biomarkers have also been implemented in the revised diagnostic criteria of AD. The combination of the CSF biomarkers Aβ_1-42, T-tau and P-tau_181P results in high levels of sensitivity, specificity and diagnostic accuracy for discriminating AD from controls (including psychiatric disorders like depression). These biomarkers can be applied for diagnosing AD in the prodromal phase of the disease (mild cognitive impairment). In case of doubt between vascular dementia (VaD) or mixed AD-VaD pathology in dementia patients, the determination of CSF Aβ_1-42, T-tau and P-tau_181P levels is of help to confirm or exclude the AD component in the pathophysiology of the dementia syndrome. However, their discriminatory power for the differential diagnosis of dementia is suboptimal. Other CSF biomarkers like Aβ_1-40, and those that are reflective of the pathology of non-AD dementias, could improve the accuracy of differential dementia diagnosis. The added differential diagnostic value of the CSF biomarkers Aβ_1-42, T-tau and P-tau_181P could lie within those cases in which the routine clinical diagnostic work-up is not able to discriminate between AD or non-AD dementias. In summary, the CSF biomarkers Aβ_1-42, T-tau and P-tau_181P can be used in clinical practice to discriminate AD from healthy aging (including psychiatric disorders like depression), to diagnose AD in its prodromal phase or in atypical forms with prominent non-memory impairment, to identify AD in patients with mixed pathologies and in case of an ambiguous (AD versus non-AD) dementia diagnosis.