Measurement of anti SARS-CoV-2 RBD IgG in saliva: validation of a highly sensitive assay and effects of the sampling collection method and correction by protein

Abstract

Objectives: To develop and evaluate a new highly sensitive assay to detect IgG anti-SARS-CoV-2 RBD in saliva samples. Methods: A two-step sandwich type immunoassay based on the amplified luminescent proximity homogeneous technology was developed and an analytical validation was performed. As a part of this validation, the influence of factors, such as different sampling conditions (stimulated saliva and passive drool) and the correction of values by total protein content, in the ability of saliva to detect increases in antibodies after an immune stimulus and be an alternative to serum, was evaluated. For this purpose, paired samples of saliva and serum at different times after vaccination were used. Results: Saliva concentrations were lower than serum, but both fluids showed similar kinetics, with higher correlations when saliva was obtained by passive flow and the results were not corrected by protein. Conclusions: The developed method showed a good analytical performance and can properly measure antibody concentrations in saliva of vaccinated individuals. However, saliva could have a lower sensitivity compared to serum at initial stages of the immune response and also when the antibody response decreased after a stimulus.