Analytical Performance and Evaluation in Clinical Cohorts of a Fully Automated Immunoassay for Plasma Glial Fibrillary Acidic Protein

Background/Objectives: This study aimed to perform analytical validation and evaluation of the clinical performance of the Access Glial Fibrillary Acidic Protein (GFAP) research-use-only (RUO) immunoassay (Beckman Coulter) in plasma. Methods: A fully automated GFAP immunoassay was developed and evaluated for analytical validity and clinical performance. Analytical validation assessed precision, sensitivity, linearity, analytical specificity, and the stability of calibrators and samples. Evaluation of predefined clinical performance criteria included method comparison against the Quanterix Simoa GFAP Advantage Plus (RUO) assay and assessment of GFAP levels between participants with Alzheimer’s disease (AD) and healthy controls. Results: The Access GFAP (RUO) immunoassay met all analytical criteria. Precision yielded coefficients of variation (CVs) < 10% across all concentration ranges. Sensitivity parameters included a lower limit of quantification (LLoQ) of 0.083 pg/mL and an analytical measurement range of 0.083–640 pg/mL. Linearity demonstrated <9% deviation across the measurement range. Interference testing showed <7% deviation for cross-reactants, common medications and AD-specific therapeutics. Plasma samples remained stable over 48 h at room temperature (<4% deviation) and five freeze–thaw cycles (<9% deviation). Method comparison demonstrated strong correlation with Simoa GFAP (R = 0.945) but systematic proportional bias, yielding 3% (slope = 0.030) of Simoa values. Samples from participants with AD exhibited significantly elevated GFAP levels (median 9.0 pg/mL, IQR: 5.3–19.1) versus controls (median 3.4 pg/mL, IQR: 1.5–6.5; p < 0.001). Conclusions: The high-throughput Access GFAP (RUO) immunoassay achieved all analytical performance criteria and demonstrated differences in GFAP levels between AD and healthy control sample cohorts. These findings support its use in research settings and as a foundation for future clinical implementation and may inform future studies evaluating cross-platform harmonization and potential clinical applications.