Evaluating the serological status of COVID-19 patients using an indirect immunofluorescent assay, France
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ORIGINAL ARTICLE
Evaluating the serological status of COVID-19 patients using an indirect immunofluorescent assay, France S. Edouard 1,2 & P. Colson 1,2 & C. Melenotte 1 & F. Di Pinto 1 & L. Thomas 1 & B. La Scola 1,2 & M. Million 1,2 & H. Tissot-Dupont 1 & P. Gautret 1,3 & A. Stein 1,2 & P. Brouqui 1,2 & P. Parola 1,3 & J.-C. Lagier 1,2 & D. Raoult 1,2 & Michel Drancourt 1,2 Received: 24 September 2020 / Accepted: 5 November 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract An indirect in-house immunofluorescent assay was developed in order to assess the serological status of COVID-19 patients in Marseille, France. Performance of IFA was compared to a commercial ELISA IgG kit. We tested 888 RT-qPCR-confirmed COVID-19 patients (1302 serum samples) and 350 controls including 200 sera collected before the pandemic, 64 sera known to be associated with nonspecific serological interference, 36 sera from non-coronavirus pneumonia and 50 sera from patient with other common coronavirus to elicit falsepositive serology. Incorporating an inactivated clinical SARS-CoV-2 isolate as the antigen, the specificity of the assay was measured as 100% for IgA titre ≥ 1:200, 98.6% for IgM titre ≥ 1:200 and 96.3% for IgG titre ≥ 1:100 after testing a series of negative controls. IFA presented substantial agreement (86%) with ELISA EUROIMMUN SARS-CoV-2 IgG kit (Cohen’s Kappa = 0.61). The presence of antibodies was then measured at 3% before a 5-day evolution up to 47% after more than 15 days of evolution. We observed that the rates of seropositivity as well as the titre of specific antibodies were both significantly higher in patients with a poor clinical outcome than in patients with a favourable evolution. These data, which have to be integrated into the ongoing understanding of the immunological phase of the infection, suggest that detection anti-SARS-CoV-2 antibodies is useful as a marker associated with COVID-19 severity. The IFA assay reported here is useful for monitoring SARS-CoV-2 exposure at the individual and population levels.
Introduction The SARS-CoV-2 is a coronavirus belonging to the genus Betacoronavirus that emerged in humans in December 2019 [1]. It was first described in China before spreading and being classified as a pandemic [2]. It causes a respiratory disease known as COVID-19 that is usually mild but can result in a severe and even life-threatening pneumonia, particularly in elderly people [2–4]. On 21 September 2020, 31,033,397 SARSCoV-2 infections and 960,736 associated deaths had been reported worldwide (https://coronavirus.jhu.edu/map.html). To date, the virological diagnosis of infections by SARSCoV-2 has been essentially based on real-time reverse transcription PCR [5]. This virus has been shown to elicit specific * Michel Drancourt [email protected]
antibodies during the course of infection [6, 7]. This serological response has mainly been analysed using enzyme-linked (ELISA) or chemiluminescence immunoassays among exposed populations in China and neighbouring count
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