Concordance between the updated Elecsys cerebrospinal fluid immunoassays and amyloid positron emission tomography for Alzheimer's disease assessment: findings from the Apollo study.

Objectives: The Apollo study was designed to support the clinical performance verification of the adjusted cutoffs of the Elecsys β-Amyloid(1-42) (Aβ) cerebrospinal fluid (CSF) II, β-Amyloid(1-40) (Aβ) CSF, Phospho-Tau (181P) (pTau) CSF and Total-Tau (tTau) CSF immunoassays (Roche Diagnostics International Ltd) for measuring fresh CSF samples, and assess the concordance of the Elecsys CSF pTau/Aβ, tTau/Aβ and Aβ/Aβ ratios, as well as Aβ alone, with amyloid positron emission tomography (PET) visual read status.

Methods: The primary study endpoint was to assess the concordance of the Elecsys CSF ratios and Aβ alone with amyloid PET visual read status using fresh CSF samples collected from individuals with subjective cognitive decline or mild cognitive impairment, handled with a new routine-use pre-analytical procedure and measured with the Elecsys CSF immunoassays. The sample stability after 1- to 13-week storage at -20 °C was also investigated in an exploratory analysis.

Results: Of 108 screened individuals, 91 met the eligibility criteria, of whom 44.0 % were amyloid PET-positive and 56.0 % amyloid PET-negative. Positive percent agreement (PPA) and negative percent agreement, respectively, were 0.800 and 0.882 for pTau/Aβ, 0.775 and 0.902 for tTau/Aβ, and 0.950 and 0.824 for Aβ/Aβ. For Aβ, PPA was 0.975 and negative likelihood ratio was 0.039. Overall, 33 samples (36.3 %) were frozen at -20 °C for 1-13 weeks. All concentration recoveries were within 100 ± 10 % when stored at -20 °C for ≤8 weeks.

Conclusions: Elecsys CSF ratios and Aβ alone may be reliable alternatives to amyloid PET for identifying amyloid positivity in clinical practice.