Correlation of the commercial anti-SARS-CoV-2 receptor binding domain antibody test with the chemiluminescent reduction neutralizing test and possible detection of antibodies to emerging variants

Morinaga, Y.; Tani, H.; Terasaki, Y.; Nomura, S.; Kawasuji, H.; Shimada, T.; Igarashi, E.; Saga, Y.; Yoshida, Y.; Yasukochi, R.; Kaneda, M.; Murai, Y.; Ueno, A.; Miyajima, Y.; Fukui, Y.; Nagaoka, K.; Ono, C.; Matsuura, Y.; Fujimura, T.; Ishida, Y.; Oishi, K.; Yamamoto, Y.

Background Serological tests are beneficial for recognizing the immune response against SARS-CoV-2. To identify protective immunity, optimization of the chemiluminescent reduction neutralizing test (CRNT), using pseudotyped SARS-CoV-2, is critical. Whether commercial antibody tests are comparably accurate is unknown. Methods Serum samples collected before variants were locally found were obtained from confirmed COVID-19 patients (n = 74), confirmed non-COVID-19 individuals (n = 179), and unscreened individuals (suspected healthy individuals, n = 229). The convalescent phase was defined as the period after day 10 from disease onset. The CRNT against pseudotyped viruses displaying the wild-type spike protein and a commercially available anti-receptor binding domain (RBD) antibody test were assayed. The CRNT was also assayed, using South African (SA) and United Kingdom (UK)-derived variants. Results The CRNT (cut off value, 50% inhibition) and the anti-RBD antibody test (cut off value, 0.8 U/mL) concurred regarding symptomatic COVID-19 patients in the convalescent phase and clearly differentiated between patients and suspected healthy individuals (sensitivity; 95.8% and 100%, specificity; 99.1% and 100%, respectively). Anti-RBD antibody test results correlated with neutralizing titer (r = 0.47, 95% CI 0.20-0.68). Compared with the wild-type, CRNT reduction was observed for the SA and UK-derived variants. Of the samples with [≥]100 U/mL by the anti-RBD antibody test, 77.8% and 88.9% showed [≥]50% neutralization against the UK and the SA variants, respectively. Conclusion The CRNT and commercial anti-RBD antibody test effectively classified convalescent COVID-19 patients. The strong positive results using the commercial antibody test can reflect neutralizing activity against emerging variants.