Variability among assays complicates serological survey interpretation

Publication title: SARS-CoV-2 seroprevalence survey estimates are affected by anti-nucleocapsid antibody decline

Researchers from Public Health Ontario and the University of Toronto’s Dalla Lana School of Public Health found that there was a drop in the number of samples with detectable antibodies over time using the Abbott Architect SARS-CoV-2 IgG assay.

Residual sera, plasma and blood samples were collected at five time points in Ontario to determine the presence of antibodies against SARS-CoV-2. Two separate tests were employed. First, the Abbott Architect SARS-CoV-2 IgG positive samples detected with this test were re-tested with the Ortho-Clinical Diagnostics VITROS Anti-SARS-CoV-2 IgG test. A total of 21,676 samples were tested over five months, and overall, 1.1% were positive with both tests.

When looking at results from only the Abbott test, there was a drop in the number of positive samples over time. This effect was not observed with the Ortho test. The authors of this study suggest that these findings have an impact on the results of antibody studies which only use the Abbott test, especially in places where there is a high case load. The decline in positivity may lead to the incorrect conclusion that there are fewer people with antibodies than there really are. In order to overcome this problem, Bolontin and colleagues suggest lowering the cut-off used with the Abbott test, although this affects the characteristics of the test, or switching to another test that measures the same target.

The study provides evidence that there exists potentially significant variability in assays, which may give the impression of waning protection against COVID infection.

Bolotin S, Tran V, Osman S, Brown KA, Buchan SA, Joh E, Deeks SL, Allen VG. SARS-CoV-2 seroprevalence survey estimates are affected by anti-nucleocapsid antibody decline. J Infect Dis. 2021 Jan 5:jiaa796. doi: 10.1093/infdis/jiaa796https://pubmed.ncbi.nlm.nih.gov/33400794/#affiliation-1