Comparison of a rapid immunochromatographic test with a chemiluminescence immunoassay for detection of anti-SARS-CoV-2 IgM and IgG

Introduction The 2019 Coronavirus disease (COVID-19) has been characterized as a pandemic, representing a serious global public health emergency. Serological tests have been proposed as reliable tools for detecting Coronavirus SARS-CoV-2 antibodies in infected patients, especially for surveillance or epidemiological purposes. The aim of this study is to evaluate the agreement between the IgM/IgG rapid assays, based on lateral flow immunochromatographic assay, and the fully automated 2019-nCoV IgM and IgG, based on chemiluminescence immunoassay. Materials and methods SARS-CoV-2 antibodies were measured with the BIOSYNEX COVID-19 BSS IgM/IgG test (BIOSYNEX, Illkirch-Graffenstaden, France) and the MAGLUMI CLIA (IgM and IgG) (SNIBE – Shenzhen New Industries Biomedical Engineering, Shenzhen, China) in 70 serum samples from patients with PCR-confirmed diagnosis. The strength of the agreement of the two methods was calculated by using the Cohen Kappa index. Results The results showed a good grade of concordance between the two immunoassays with a Cohen’s kappa coefficient of 0.71 (95%CI: 0.54-0.87) for IgG SARS-CoV-2 antibodies and 0.70 (95%CI: 0.53-0.87) for IgM SARS-CoV-2 antibodies. In addition, the rapid assays BIOSYNEX COVID-19 BSS for detecting SARS-CoV-2 antibodies showed a positive likelihood ratio (LR) of 10.63 (95%CI: 2.79-40.57) for IgG and a LR of 6.79 (95%CI: 2.93-15.69) for IgM. Conclusion Our results suggest that the immunochromatographic rapid IgM/IgG test and the chemiluminescence IgM and IgG immunoassay have a good degree of concordance, suggesting that both could be considered as useful tools for epidemiologic surveillance.

1 Among infected patients who may display different clinical symptoms, ranging from mild to severe conditions, a discrete amount of individuals remains asymptomatic, being a hidden vehicle, which spreads infection. Accordingly, the World Health Organization (WHO) has stressed the need Gambino CM. et al. anti-SARS-CoV-2 IgM and IgG detection for research on in vitro rapid and effective screening test, in order to quickly identify SARS-CoV-2 infected individuals and prevent contagion (3).
To date, the diagnosis of COVID-19 must be confirmed by a molecular test using real-time reverse transcriptase-polymerase chain (RT-PCR) assay on the oropharyngeal or nasopharyngeal swabs for detecting 2019-nCoV genome (4). Although RT-PCR represents the gold standard for COVID-19 diagnosis, serological tests are emerging as valid tools for identifying infected subjects, especially for surveillance or epidemiological purposes (5).
Although there are many commercially available COVID-19 antibody tests, data concerning the performance of these assays are currently limited, and many of these tests have been approved for research use only.

Subjects
A total of 70 patients with PCR-confirmed COV-ID-19 diagnosis (41 males and 29 females, median age 66 (59-74) years), from April to May 2020, were enrolled in this study. For each patient, the serum sample was obtained during the hospital stay, 7-10 days after nasopharyngeal and oropharyngeal swabs collection. The SARS-CoV-2 IgM and IgG measurement was performed at the Department of Laboratory Medicine, University Hospital "P. Giaccone", Palermo, Italy. All samples were analysed anonymously. Demographical and clinical data were recorded at admission. All the clinical and biological assessments were carried out in accord-ance with the Declaration of Helsinki, and the study was approved by the local Ethics Committee. All participants gave written consent.

CLIA assay
The MAGLUMI 2019-nCoV IgG/IgM assay is a capture CLIA for the quantitative measurement of IgM and IgG antibodies against SARS-CoV-2 in human serum by using the fully automated MAGLUMI analyser (SNIBE -Shenzhen New Industries Biomedical Engineering, Shenzhen, China). According to the manufacturer's instructions, serum antibodies directed against both virus spike (CoV-S) and nucleocapsid (CoV-N) are detected by using magnetic microbeads coated with ABEI-labelled 2019-nCoV recombinant antigen. As declared by the manufacturer, a threshold of positivity of 1.00 AU/ ml was established for both IgM and IgG, whilst the overall indicated reproducibility of assays ranges from 6.8% to 8.7%. Borderline data (≥ 0.95 to < 1.0) were considered reactive.

Immunochromatographic assay
The BIOSYNEX COVID-19 BSS assay (BIOSYNEX, Illkirch-Graffenstaden, France) is a rapid, portable qualitative chromatographic assay. The test cassette consists of SARS-CoV-2 recombinant antigens (Spike Protein, as declared by manufacturer) conjugated with colloidal gold; a nitrocellulose membrane strip containing an IgG line (G Line) coated with anti-human IgG, an IgM line (M Line) coated with anti-human IgM, and the control line (C Line) coated with goat-anti-rabbit IgG. It was performed on serum samples according to the manufacturer's instructions. If a line was observed for IgM and/or IgG, the test was considered positive. If the control line did not appear, the test was invalidated and repeated. Weak signal for IgM and IgG, together or separate, was considered positive.
Ten serum samples were randomly selected and tested six times in order to evaluate the assay repeatability. Two independent operators observed repeatability of 0.83. Both tests are CE (European Community) approved.  (6).

Results
The results of the comparison between rapid BIO-SYNEX COVID-19 BSS with the MAGLUMI 2019-nCoV for SARS-CoV-2 IgM and IgG detection are reported in Table 1.

Discussion
In the current study, we compared the results of the rapid test cassette BIOSYNEX COVID-19 BSS for detecting SARS-CoV-2 IgM and IgG with the fully automated MAGLUMI 2019-nCoV CLIA, used as the reference method. Although the MAGLUMI 2019-nCoV assay cannot be considered the serological gold standard test, its analytical performance has been successfully evaluated by Lippi et al. and Padoan et al. (7,8). The authors reported a sensitivity of 100% and 88% for detecting SARS-CoV-2 IgG and IgM in COVID-19 patients (7,8).
In the current study, we showed that the BIOSYN-EX COVID-19 BSS IgM/IgG rapid test cassette is congruent with the fully automated MAGLUMI 2019-nCoV, in detecting SARS-CoV-2 antibodies.
Our findings are in accordance with Hoffman et al. (9). Thus, the BIOSYNEX COVID-19 BSS IgM/IgG rapid test cassette could be used as a quick tool for possible identification of subjects who have had exposure to SARS-CoV-2 infection and thus developed antibodies. Importantly, the target of antibodies detected by BIOSYNEX assay is the viral spike protein binding the spike protein's receptorbinding domain. As known, any antibody cross-reactivity between common cold human Coronavirus and SARS-CoV-2 would result in false-positive interfering with antibody-based testing and SARS-CoV-2 surveillance. Scientific evidence suggests that the overall specificity of serological assays using only the nucleocapsid protein is poor, whereas assays based on the spike protein are more specific (10 Serological tests are useful potential tools for the rapid screening population, helping to prevent contagion. They are important for activating serological surveillance at the local, regional, and national levels and identifying people who have already had contact with the virus. Compared to molecular assays, these methods save time and are simple to perform, playing an essential role in large-scale testing to evaluate people's immunity against SARS-CoV2. However, it should be noted that there is no certainty whether people having antibodies are protected from a second infection.
Some limitations shall be disclosed in this study. First, clinical data were not available for all patients due to the limited study period. Second, we used MAGLUMI 2019-nCoV assay as a reference method since no relevant gold standard for serological assays is currently validated for comparative studies. Finally, another limitation is the lack of estimation of the prevalence of the infected population. This is due to the severe Coronavirus situation at the time of the study.
In conclusion, our results suggest that the immunochromatographic rapid BIOSYNEX COVID-19 BSS IgM/IgG test has a good degree of concordance with the MAGLUMI CLIA (IgM and IgG). Thus, it might be considered as an efficient additional tool for characterizing subjects who have had a prior infection and thus developed SARS-CoV-2 antibodies.

Potential conflict of interest
None declared.