Laboratory investigations of serum antibody are important for the diagnosis of respiratory pathogen infection. Lateral flow assay based on the gold-based immunochromatographic assay is a very important tool to detect serum antibody, and has some advantages, including rapid testing, easy operation, and no need for an apparatus or site limitations. The lateral flow assay has been used for the diagnosis of many pathogens [
12‐
15]. Because of the difficulty in obtaining natural antigen, recombinant protein has become the main diagnostic antigen for immunoassay for the diagnosis of pathogenic microorganism infection [
11,
16,
17]. In this study, we successfully expressed N protein and S1 fragment of SARS-CoV-2 in
E. coli, and developed a rapid antibody test kit and IgM assay for the diagnosis of SARS-CoV-2 infection using recombinant N protein and S1 fragment, respectively. The antibody test results from COVID-19 patients using the rapid antibody test with recombinant N protein had higher analytical sensitivity and specificity (88.56 % and 97.42 %) compared with the IgM assay with the recombinant S1 fragment (40.74 % and 93.11 %). There are several possible reasons for the lower sensitivity and specificity of the antibody IgM assay. First, the IgM assay only tested sera IgM antibody, and sera IgM levels in the majority of patients after infection are below the detection limit of the assay. However, the rapid antibody test with recombinant N protein could test total antibodies in the sera including IgA, IgM, and IgG. Second, the immune systems of patients have different antibody responses for various viral proteins (e.g., S, N, M, and E protein) after infection and have a stronger antibody response for N protein. Third, rheumatoid factors can cause false-positive results in the sera IgM assay, but the rapid antibody test kit does not have this issue because it is developed with a double-antigen sandwich. Therefore, the rapid antibody test kit with recombinant N protein is suitable for SARS-CoV-2 infection diagnosis. To evaluate the application of the rapid antibody test kit, 97 patients with COVID-19, 58 health professionals, 88 patients with general fever or pulmonary infection, and 48 doctors and nurses who worked in the fever outpatient clinic were analyzed, and the results demonstrated that it had 88.56 % sensitivity and 97.42 % specificity. Sensitivity of the rapid antibody test kit was in agreement with a recently reported kit, but its specificity was higher than that of the reported one, which is a rapid IgM-IgG combined antibody test kit developed using a recombinant receptor binding domain (RBD) of SARS-CoV-2 S protein with 88.66 % sensitivity and 90.3 % specificity [
18]. The specificity difference of the two kits may be due to their antigens and preparation technology, because the rapid antibody test kit was developed using recombinant N protein and prepared by double-antigen sandwich, and the reported IgM-IgG combined antibody test kit was developed using the recombinant RBD of the S protein and prepared by the immunocapture method.
For clinical evaluation of the rapid antibody test kit in suspected COVID-19 patients, 43 suspected COVID-19 patients were analyzed. The test results showed 53.48 % positivity, which was significantly higher than the negative control (4.55 %), although significantly lower than that of COVID-19 patients (88.56 %). These results suggest that due to false-negative RT-PCR results, more than 50 % of suspected COVID-19 patients may have SARS-CoV-2 infection, and the possibility of positive cases of general fever or pulmonary infection in patients with COVID-19 cannot be ruled out. To clinically evaluate the rapid antibody test kit for COVID-19 patients at different points during disease progression, 27 COVID-19 patients at different points in their disease and 15 recovering patients were analyzed. The results demonstrated that the rapid antibody test kit had 88.88 % sensitivity on day 1 and 92.59 % sensitivity on day 10 after hospitalization in COVID-19 patients, and 57.14 % sensitivity on day 14 after recovery for recovering patients. Because the latent period of SARS-CoV-2 infection is 2−7 days, and the median is 4 days [
3], the number of COVID-19 patients who produced specific antibodies on day 10 was more than that on day 1 after hospitalization. Therefore, the positive antibody detection rate in the preliminary stage of COVID-19 was lower than that in the later stage. These results demonstrated that a few COVID-19 patients could not produce specific antibodies, and the antibody levels of some recovering patients gradually decreased until they disappeared. To determine if there was any cross-reactivity between the rapid antibody test kit with other pathogenic microorganism infections, cross-reactivity experiments were conducted. The results demonstrated that the rapid antibody test kit had low cross-reactivity (0.5−0.8 %) in patients infected with the
M. Pneumoniae respiratory pathogen. The reasons for the cross-reactivity are unclear and need further study.
There are some limitations in our research. First, the protein preparation and clearly the purity is significantly different between NiNTA of N protein and urea preparation of S1 proteins, which may affect sensitivity between peptides. Second, although immunological detection methods developed to detect virus-specific antibodies are mainly aimed at the structural proteins including S protein and N protein, it is well established that the most significant neutralization activity of antibodies produced by recovered patients is mediated by epitopes to S1 protein and the RBD domain, not N protein. Therefore, utility of N protein antibody tests will be limited in interpretation of recovering patients and of no use to vaccinated individuals.