Elecsys® Phospho-Tau (181P) CSF is an in vitro diagnostic immunoassay intended for the quantitative determination of the pTau 181 (Tau protein phosphorylated at threonine 181) concentration in human cerebrospinal fluid (CSF). Paired with the Elecsys® β-Amyloid (1-42) CSF II assay, the pTau181/Abeta42 ratio measures two main biomarkers of Alzheimer’s pathology, beta-amyloid 42 and phosphorylated tau 181 proteins, in adults ages 55 and older being evaluated for AD or other causes of cognitive impairment.1
The abnormal phosphorylation of tau protein leads to formation of intracellular neurofibrillary tangles. Together with the amyloid plaques, these modifications constitute the hallmark pathologies in AD and changes in these biomarkers were shown to occur before clinical symptoms.2,3 Studies demonstrated that while Abeta42 levels in CSF decrease to around half the level in controls, CSF pTau181 levels increase around 2‑3 fold in mild‑moderate AD patients compared to age‑matched controls.4,5
Biomarker confirmation of amyloid positivity either by amyloid PET or CSF testing is one of the requirements listed in the appropriate use recommendations for amyloid-targeting therapies.6
The Elecsys® pTau181 CSF assay is not intended to be used as a stand-alone test and should only be used with the Elecsys® β-Amyloid (1-42) CSF assay to calculate the ratio of Phospho-Tau (181P) (pTau181) to β-Amyloid (1-42) (Abeta42) in CSF (pTau181/Abeta42 ratio).1
A negative result, defined as pTau181/Abeta42 ratio value below cutoff or an Abeta42 value above the measuring range, is consistent with a negative amyloid positron emission tomography (PET) scan result and reduces the likelihood that a patient’s cognitive impairment is due to AD. A positive result, defined as pTau181/Abeta42 ratio value above cut‑off, is consistent with a positive amyloid PET scan result.1
Although a positive result does not establish a diagnosis of AD or other cognitive disorder, the use of the pTau181/Abeta42 ratio result in addition to other clinical diagnostic evaluations enhances diagnosis accuracy and can streamline patient management.1,7,8