AbstractINTRODUCTIONBlood biomarkers have proven useful in Alzheimer's disease (AD) research. However, little is known about their biological variation (BV), which improves the interpretation of individual‐level data.METHODSWe measured plasma amyloid beta (Aβ42, Aβ40), phosphorylated tau (p‐tau181, p‐tau217, p‐tau231), glial fibrillary acidic protein (GFAP), and neurofilament light chain (NfL) in plasma samples collected weekly over 10 weeks from 20 participants aged 40 to 60 years from the European Biological Variation Study. We estimated within‐ (CV_I) and between‐subject (CV_G) BV, analytical variation, and reference change values (RCV).RESULTSBiomarkers presented considerable variability in CV_I and CV_G. Aβ42/Aβ40 had the lowest CV_I (≈ 3%) and p‐tau181 the highest (≈ 16%), while others ranged from 6% to 10%. Most RCVs ranged from 20% to 30% (decrease) and 25% to 40% (increase).DISCUSSIONBV estimates for AD plasma biomarkers can potentially refine their clinical and research interpretation. RCVs might be useful for detecting significant changes between serial measurements when monitoring early disease progression or interventions.Highlights Plasma amyloid beta (Aβ42/Aβ40) presents the lowest between‐ and within‐subject biological variation, but also changes the least in Alzheimer's disease (AD) patients versus controls. Plasma phosphorylated tau variants significantly vary in their within‐subject biological variation, but their substantial fold‐changes in AD likely limits the impact of their variability. Plasma neurofilament light chain and glial fibrillary acidic protein demonstrate high between‐subject variation, the impact of which will depend on clinical context. Reference change values can potentially be useful in monitoring early disease progression and the safety/efficacy of interventions on an individual level. Serial sampling revealed that unexpectedly high values in heathy individuals can be observed, which urges caution when interpreting AD plasma biomarkers based on a single test result.