Abstract
PCR methods are presently the standard for the diagnosis of Coronavirus disease 2019 (COVID-19), but additional methodologies are needed to complement PCR methods, which have some limitations. Here, we validated and investigated the usefulness of measuring serum antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) using the iFlash3000 CLIA analyzer. We measured IgM and IgG titers against SARS-CoV-2 in sera collected from 26 PCR-positive COVID-19 patients, 53 COVID-19-suspected but PCR-negative patients, and 20 and 100 randomly selected non-COVID-19 patients who visited our hospital in 2020 and 2017, respectively. The repeatability and within-laboratory precision were obviously good in validations, following to the CLSI document EP15-A3. Linearity was also considered good between 0.6 AU/mL and 112.7 AU/mL for SARS-CoV-2 IgM and between 3.2 AU/mL and 55.3 AU/mL for SARS-CoV-2 IgG, while the linearity curves plateaued above the upper measurement range. We also confirmed that the seroconversion and no-antibody titers were over the cutoff values in all 100 serum samples collected in 2017. These results indicate that this measurement system successfully detects SARS-CoV-2 IgM/IgG. We observed four false-positive cases in the IgM assay and no false-positive cases in the IgG assay when 111 serum samples known to contain autoantibodies were evaluated. The concordance rates of the antibody test with the PCR test were 98.1% for SARS-CoV-2 IgM and 100% for IgG among PCR-negative cases and 30.8% for SARS-CoV-2 IgM and 73.1% for SARS-CoV-2 IgG among PCR-positive cases. In conclusion, the performance of this new automated method for detecting antibody against both N and S proteins of SARS-CoV-2 is sufficient for use in laboratory testing.
Highlights
In December 2019, the first pneumonia cases caused by an unknown microorganism were identified in Wuhan, China [1]
We performed the verification of repeatability and within-laboratory precision according to the Clinical & Laboratory Standards Institute (CLSI) document EP15-A3 for five days and with five replicates per run, using two manufacturer’s controls each assay (Table 1)
When we investigated linearity using samples with moderate antibody titers, the curves showed a good linearity between 0.6 arbitrary unit (AU)/mL and 112.7 arbitrary units per milliliter (AU/mL) for SARS-CoV-2 immunoglobulin M (IgM) and between 3.2 AU/mL and 55.3 AU/mL for SARS-CoV-2 immunoglobulin G (IgG) (Fig 1A–1D)
Summary
In December 2019, the first pneumonia cases caused by an unknown microorganism were identified in Wuhan, China [1]. The pathogen was identified as a novel betacoronavirus and was named “severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)” [2]. SARS-CoV-2 is phylogenetically similar to SARS-CoV, which caused outbreaks of a severe respiratory syndrome in China in 2002 [3]. The symptoms of coronavirus disease 2019 (COVID-19), which is the respiratory syndrome caused by SARS-CoV-2, are fever, cough and lymphopenia [4]. Since early December 2019 and as of June 15, 2020, over 7,800,000 cases of COVID-19 have been confirmed and 430,000 deaths have been reported throughout the world [6], and the World Health Organization has reported a fatality rate for cases defined as pneumonia of approximately 2% [7]
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