Direct immunoassay on a polyester microwell plate for colorimetric detection of the spike protein in swab and saliva samples

Abstract

This study presents the development of a polyester microplate for detecting the S-protein of the SARS-CoV-2 virus in saliva and nasopharyngeal swab samples using direct enzyme-linked immunosorbent assay (ELISA) technology. The polyester microplate was designed to contain 96 zones with a 3 mm diameter each, and a volume of 2–3 μL. The experimental conditions including reagent concentration and reaction time were optimized. The microplate image was digitized and analyzed using graphical software. The linear range obtained between protein S concentrations and pixel intensity was 0–10 μg mL⁻¹, with a correlation coefficient of 0.99 and a limit of detection of 0.44 μg mL⁻¹. The developed methodology showed satisfactory intraplate and interplate repeatability with RSD values lower than 7.8%. The results achieved through immunoassay performed on polyester microplates were consistent with those of the RT-PCR method and showed a sensitivity of 100% and 90% and specificity of 85.71% and 100% for saliva and nasopharyngeal samples, respectively. The proposed direct immunoassay on polyester microplates emerges as an alternative to conventional immunoassays performed on commercial polystyrene plates, given the low cost of the device, low consumption of samples and reagents, lower waste generation, and shorter analysis time. Moreover, the immunoassay has shown great potential for diagnosing COVID-19 with precision and accuracy.