Parallel detection of SARS-CoV-2 RNA and nucleocapsid antigen in nasopharyngeal specimens from a COVID-19 patient screening cohort

Mayanskiy, Nikolay; Brzhozovskaya, Ekaterina; Fedorova, Natalia; Lebedin, Yuri

Objectives Reverse-transcription PCR (RT-PCR) is considered the most sensitive method in detecting severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2). However, this method is relatively resource- and time-consuming. Here, we compared SARS-CoV-2 nucleocapsid antigen (N-Ag) testing using an enzyme-linked immunosorbent assay (ELISA) with SARS-CoV-2 RNA detection. Methods Parallel SARS-CoV-2 RT-PCR and quantitative N-Ag ELISA analysis was executed in nasopharyngeal specimens obtained during SARS-CoV-2 screening in a cohort of pre-hospitalization patients. Results In total, 277 specimens were examined, including 182 (65.7%) RT-PCR-positive specimens, which demonstrated a median cycle threshold (Ct) value of 27 (interquartile range [IQR], 23-35). The SARS-CoV-2 N-Ag was detected in 164/182 RT-PCR-positive specimens (overall sensitivity, 90.1%). Among 95 RT-PCR-negative specimens, 72 were N-Ag-negative (specificity, 75.8%). SARS-CoV-2 RT-PCR and N-Ag ELISA results demonstrated a strong agreement (Cramer's V, 0.668; p < 0.001). N-Ag concentrations spanned from 5.4 to 296,000 pg/mL (median, 901 pg/mL; IQR, 43-1407 pg/mL) and were inversely correlated with Ct values (Spearman's r=-0.720; p < 0.001). Conclusions SARS-CoV-2 N-Ag ELISA results were in close agreement with RT-PCR results, and N-Ag concentrations were proportional to viral loads. Thus, SARS-CoV-2 quantitative antigen testing could be an additional diagnostic instrument for SARS-CoV-2.