Clinical Performance of the Elecsys CSF pTau₁₈₁/Aβ₄₂ Ratio for Concordance with Tau-PET in Two Independent Cohorts

Amyloid- and tau-positron emission tomography (PET) are promising modalities for detecting pathological changes associated with Alzheimer's disease (AD); however, their application is limited. Although the use of cerebrospinal fluid (CSF) biomarkers as alternatives to amyloid-PET and tau-PET has been explored, no in vitro diagnostic-approved or commercial CSF biomarker assays are currently available for detecting tau pathology in clinical practice.

In this study, we determined and validated the optimal cutoff value for the ratio of tau phosphorylated at a threonine residue at position 181 (pTau₁₈₁) to β-amyloid(1-42) (Aβ₄₂) in CSF, measured with the Elecsys^® Phospho-Tau (181P) CSF and β-Amyloid(1-42) CSF immunoassays (Roche Diagnostics International Ltd, Rotkreuz, Switzerland), based on its concordance with binary tau-PET status. Clinical performance was explored using CSF measurements and tau-PET scans retrospectively obtained from a subset of subjects with mild cognitive impairment and dementia due to AD in two independent cohorts, Alzheimer's Disease Neuroimaging Initiative-2/3 (ADNI-2/3; N = 133) and Swedish BioFINDER-2 (N = 62).

In the first part of this analysis (ADNI-2/3 pre-analytics), a CSF pTau₁₈₁/Aβ₄₂ cutoff value of 0.0395 was selected as the best compromise between positive percent agreement (PPA) and negative percent agreement (NPA) for the tau-PET visual read endpoint. After adjustment to account for differences between the ADNI-specific protocol and the manufacturer's recommended pre-analytical protocol used in BioFINDER-2, the optimal CSF pTau₁₈₁/Aβ₄₂ cutoff value was set at 0.037. The adjusted cutoff was validated in BioFINDER-2 and was associated with a PPA of 85.7% (95% confidence interval [CI] 70.6, 93.7), NPA of 70.4% (95% CI 51.5, 84.1), and overall percent agreement (OPA) of 79.0% (95% CI 67.4, 87.3); the positive and negative likelihood ratios were 2.89 and 0.203, respectively.

The Elecsys CSF pTau₁₈₁/Aβ₄₂ ratio may be a reliable tool for identifying tau pathology in clinical practice.