Evaluation of production lots of a rapid point-of-care lateral flow serological test intended for identification of IgM and IgG against the N-terminal part of the spike protein (S1) of SARS-CoV-2
Abstract
Background and objectives
Several antibody tests are available to detect SARS-CoV-2 specific antibodies, many of which address different antigens. Rapid point-of-care (POC) tests have been doubted due to an eventual risk of production errors, although it is unstudied whether such error would affect test sensitivity and/or specificity. We aimed to evaluate two separate production lots of a commercially available test intended for rapid detection of IgM and IgG against the N-terminal part of the SARS-CoV-2 spike protein (S1).
Materials and methods
Serum samples from individuals with confirmed SARS-CoV-2 infection, by RT-PCR and/or serology, and pre-COVID-19 negative control sera gathered from a biobank during 2018 were collected. The presence of anti-S1 IgM/IgG was verified by an in-house Luminex-based serological assay, serving as reference method. The index test was a commercially available rapid POC-test (the COVID-19 IgG/IgM Rapid Test Cassette [Zhejiang Orient Gene Biotech Co Ltd, Huzhou, Zhejiang, China/Healgen Scientific, LLC, U.S.A.]).
Results
One hundred samples were verified positive for anti-S1 IgG (median fluorescence intensity (MFI) ≥900) and 74 for anti-S1 IgM (MFI ≥700), confirmed by RT-PCR (n=90) and/or serology (n=89). None of the negative controls (n=200; MFI <300) had SARS-CoV-2 anti-S1 IgM, while one tested positive for SARS-CoV-2 anti-S1 IgG. For the two lots, the sensitivities of the rapid test were 93.2% (69/74; 95% CI: 85.1% – 97.1%) and 87.8% (65/74; 95% CI: 78.5% – 93.5%) for IgM, respectively 93.0% (93/100; 95% CI: 86.3% – 96.6%) and 100.0% for IgG (100/100; 95% CI: 96.3% – 100.0%). The specificity for both lots was 100% for IgM (200/200; 95% CI: 98.1% – 100%) and 99.5% for IgG (199/200; 95% CI: 97.2% – 99.9%). The positive predictive value was 100% for IgM and 98.9% and 99.0% for IgG. The negative predictive value was 95.7% and 97.6% for IgM, and 96.6% and 100.0% for IgG.
Conclusion
The rapid POC-test used in this study is suitable to assess SARS-CoV-2 anti-S1 specific IgM/IgG, as a measure of previous virus exposure on an individual level. While the specificity was not affected by production lot, external validation of separate lots of rapid POC-tests is encouraged to ensure high sensitivity before market introduction.
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