Novel assay detects all COVID-19 antibody isotypes, even in asymptomatic cases

A novel, flow cytometry-based assay can detect various isotypes of antibodies against the SARS-CoV-2 spike (S) protein, both in symptomatic and asymptomatic patients, according to a recent Singapore study.

“The high sensitivity of the S-protein flow cytometry-based (SFB) assay is also particularly useful in clinical investigation of suspected infections and epidemiological link within clusters, which might yield borderline/discrepant results or even be missed by less sensitive serological assays,” the researchers said, adding that the SFB also “aids in contact tracing efforts to limit the extent of community spread.”

Eighty-one symptomatic patients with the novel coronavirus disease (COVID-19) were enrolled for testing the SFB assay, and various control groups were used for comparison: patients who had recovered from the severe acute respiratory syndrome (SARS), those who had seasonal human coronavirus infections, and healthy individuals. Antibody responses were assessed at 5, 10, and 23 days after illness onset.

IgM antibodies against the S protein were detected among the COVID-19 patients, but not in any of the control groups. IgM response strengthened with time: 32 percent of patients showed positive signals at 5 days, climbing to 63 percent and 92 percent at 10 and 23 days after onset, respectively. [Cell Rep Med 2021;doi: 10.1016/j.xcrm.2021.100193]

The assay was also sensitive enough to detect both IgA and IgG responses, both of which displayed a similar temporal pattern as IgM and strengthened with time.

IgG seroconversion could also be detected. At 5 days after onset, 47 percent of patients had signals of seroconversion, and by day 23, 100-percent seroconversion was achieved. Notably, four recovered SARS patients showed IgG cross-reactivity against the SARS-CoV-2 S protein.

Moreover, the SFB assay could also help identify asymptomatic or presymptomatic COVID-19 patients. Using samples from the Singapore National Public Health Laboratory, the researchers detected IgM, IgA, and IgG responses against the S protein in asymptomatic patients who had tested positive for COVID-19. These patients showed significantly weaker antibody responses relative to their symptomatic counterparts.

In terms of diagnostic utility, the SFB had specificity values of 100 percent, 98 percent, and 93 percent of IgM, IgA, and IgG detection, respectively, in symptomatic patients for all three time points. Sensitivity estimates, on the other hand, varied across the different assessment times. At 23 days postonset, the corresponding estimates for the respective antibody isotypes were 92 percent, 98 percent, and 100 percent.

“Taken together, our findings demonstrated that the SFB assay could be used for serological confirmation of symptomatic infections. The SFB assay could also be used, in combination with other serological assays, to detect asymptomatic infections and assess seroprevalence in the community,” the researchers said. “This would be an instrumental tool for sero-surveillance and provide crucial insight to the extent of undetected and undiagnosed COVID-19 cases in the community.”

“This is especially pertinent at a time when governments around the world are looking to gradually reopen the economy,” they added.