The accuracy of the fluorometric and colorimetric assays was assessed by studying analyte recovery from spiked biological samples. Saliva samples were spiked with S protein RBD and analyzed. As shown in Table , the average recoveries of RBD from the spiked saliva samples were 93-100% and 87.2-107% for fluorometric and colorimetric assays, respectively. The relative standard deviation (RSD) of measurements were 4.03-10.6% and 6.15-9.04%, respectively. Therefore, the RCAenabled protein assays based on fluorescence and absorbance measurements allow to detect and quantify SARS-CoV-2 RBD in untreated biological samples.