One-Step Quantification of anti-Covid-19 Antibodies via Dual Affinity Ratiometric Quenching Assays

The global pandemic caused by acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has affected millions of people and paralyzed healthcare systems worldwide. Developing rapid and accurate tests to detect and quantify anti-SARS-CoV-2 antibodies in complex fluids is critical to (i) track and address the spread of SARS-CoV-2 variants with different virulence and (ii) support the industrial manufacturing and clinical administration of anti-SARS-CoV-2 therapeutic antibodies. Conventional immunoassays, such as lateral flow, ELISA, and surface plasmon resonance (SPR), are either qualitative or, when quantitative, are laborious and expensive and suffer from high variability. Responding to these challenges, this study evaluates the performance of the Dual-Affinity Ratiometric Quenching (DARQ) assay for the quantification of anti-SARS-CoV-2 antibodies in bioprocess harvests and intermediate fractions (i.e., a Chinese hamster ovary (CHO) cell culture supernatant and a purified eluate) and human fluids (i.e., saliva and plasma). Monoclonal antibodies targeting the SARS-CoV-2 nucleocapsid as well as the spike protein of the delta and omicron variants are adopted as model analytes. Additionally, conjugate pads loaded with dried protein were studied as an at-line quantification method that can be used in clinical or manufacturing laboratories. Our results indicate that the DARQ assay is a highly reproducible (coefficient of variation ∼0.5-3%) and rapid (<10 min) test, whose sensitivity (∼0.23-2.5 ng/mL), limit of detection (23-250 ng/mL), and dynamic range (70-1300 ng/mL) are independent of sample complexity, thus representing a valuable tool for monitoring anti-SARS-CoV-2 antibodies.