Elecsys pT181p immunoassay: Clinical performance for ruling out amyloid pathology

INTRODUCTION Plasma biomarker rule‐out tests can be used early in the Alzheimer's disease (AD) diagnostic pathway to identify people with low likelihood of amyloid pathology. METHODS A care setting‐agnostic cutoff for the Elecsys ® Phospho‐Tau (181P) plasma (pT181p) immunoassay was determined and validated in two independent, diverse cohorts, representative of real‐world clinical practice ( N = 604; N = 787, respectively). An additional cutoff was assessed in a subset of people reflective of primary care ( n = 312). RESULTS At a care setting‐agnostic cutoff of 0.934 pg/ml, high negative predictive value (NPV) (93.8%) and acceptable positive predictive value (PPV) (46.6%) were observed against amyloid‐positron emission tomography (PET). A primary care‐specific cutoff of 0.722 pg/ml showed high NPV (97.9%) against amyloid‐PET. Amyloid positivity prevalence for each cutoff population was 22.5% and 13.1%, respectively. DISCUSSION The pT181p immunoassay is a high‐performance test that may substantially help improve access to amyloid pathology testing for people requiring confirmation, potentially leading to timely clinical decisions and better outcomes.