Antibody Tests Detect COVID-19 Antibodies 20 Weeks After First Symptoms

In a recent study, researchers report the persistence of SARS-CoV-2 IgG up to 140 days (20 weeks) post-infection, across three antibody immunoassays, and propose a standard for assessing SARS-CoV-2 antibody assays. The authors use a ‘pseudo gold standard’ cohort (n=348 positive, n=510 negative) to determine the sensitivity and specificity of the three commercial immunoassays i.e., EuroImmun, Roche, and Abbott.

The study cohort included 880 people in Northern Ireland. There is no clear gold standard for reference to assess SARS-CoV-2 immunoassays. RT-PCR is used as a reference standard; however, it is limited by a short temporal window for a positive diagnosis and exhibits potential for false-negative results. With lockdown measures and social-distancing strategies, many cases were largely unconfirmed or undetected. The commercial serology immunoassays are laboratory-based and measure IgG antibody levels in plasma or serum.

Alternatively, lateral flow immunoassays (LFIAs), requiring a finger-prick blood sample, can be used at home or lab care. The authors use these tests to detect antibodies generated against the S1 domain of the spike antigenic protein of SARS-CoV-2, to characterize a large number (880) of pre-pandemic and pandemic COVID-19 blood samples from within Northern Ireland. IgG antibodies to both the spike protein and nucleocapsid protein are persistent even after 140 days after RT PCR positive status; the authors report a statistically significant decline over time, but the levels remain detectable at 140 days.

A further study was assessed with sequential antibody immunoassays performed on symptomatic and asymptomatic individuals in addition to patients with mild symptoms. The results of the three test kits show a strong correlation between all three immunoassays, with shortcomings in the Abbott system. It is crucial to consider the prevalence when interpreting assays’ sensitivity and specificity in a low prevalence scenario. Even slightly lowered performance metrics can result in large numbers of false-negative and false-positive results. In conclusion, the researchers report the longevity of antibodies against the SARS-CoV-2 virus in the plasma of a large cohort of individuals – lasting up to 140 days; and they have developed a ‘pseudo gold standard’ reference cohort against which to assess immunoassay performance.

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