Evaluation of a SARS-CoV-2 Capture IgM Antibody Assay in Convalescent Sera

Abstract

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is responsible for a global pandemic with over 152 million cases and 3.19 million deaths reported by early May 2021. Understanding the serological response to SARS-CoV-2 is critical to determining the burden of infection and disease (coronavirus disease 2019 [COVID-19]) and transmission dynamics. We developed a capture IgM assay because it should have better sensitivity and specificity than the commonly used indirect assay. Here, we report the development and performance of a capture IgM enzyme-linked immunosorbent assay (ELISA) and a companion indirect IgG ELISA for the spike (S) and nucleocapsid (N) proteins and the receptor-binding domain (RBD) of S. We found that among the IgM ELISAs, the S ELISA was positive in 76% of 55 serum samples from SARS-CoV-2 PCR-positive patients, the RBD ELISA was positive in 55% of samples, and the N ELISA was positive in 15% of samples. The companion indirect IgG ELISAs were positive for S in 89% of the 55 serum samples, RBD in 78%, and N in 85%. While the specificities for IgM RBD, S, and N ELISAs and IgG S and RBD ELISAs were 97% to 100%, the specificity of the N IgG ELISA was lower (89%). RBD-specific IgM antibodies became undetectable by 3 to 6 months, and S IgM reached low levels at 6 months. The corresponding IgG S, RBD, and N antibodies persisted with some decreases in levels over this time period. These capture IgM ELISAs and the companion indirect IgG ELISAs should enhance serologic studies of SARS-CoV-2 infections. IMPORTANCE Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has inflicted tremendous loss of lives, overwhelmed health care systems, and disrupted all aspects of life worldwide since its emergence in Wuhan, China, in December 2019. Detecting current and past infection by PCR or serology is important to understanding and controlling SARS-CoV-2. With increasing prevalence of past infection or vaccination, IgG antibodies are less helpful in diagnosing a current infection. IgM antibodies indicate a more recent infection and can supplement PCR diagnosis. We report an alternative method, capture IgM, to detect serum IgM antibodies, which should be more sensitive and specific than most currently used methods. We describe this capture IgM assay and a companion indirect IgG assay for the SARS-CoV-2 spike (S), nucleocapsid (N), and receptor-binding domain (RBD) proteins. These assays can add value to diagnostic and serologic studies of coronavirus disease 2019 (COVID-19).

Keywords: COVID-19; IgG ELISA; SARS-CoV-2; antibody duration; capture IgM ELISA.

FIG 1

Comparison of the separation of P and P-N absorbance values between controls and PCR⁺ specimens for S IgM and IgG ELISAs. Controls are serum specimens collected before 2020 (n = 113; 40 pairs and 33 single serum samples), and PCR⁺ samples are 55 serum samples from 55 patients with PCR⁺ COVID-19. P is the average absorbance (2 wells/specimen) against S antigen, and P-N is P minus the average absorbance (2 wells/specimen) against control antigen. The graph shows mean values and standard deviations. Note the much clearer separation between control and PCR⁺ specimens for P-N than for P.

FIG 2

COVID-19 patient serum samples possess SARS-CoV-2-specific IgG and IgM antibodies. The P-N absorbance values for control specimens, serum samples collected before 2020, and PCR⁺ specimens, 55 serum samples from 55 patients with PCR⁺ COVID-19, are depicted. Specimens were tested at a 1:200 dilution against receptor-binding domain (RBD), spike protein (S), or nucleocapsid protein (N) antigens. Controls (n = 73; 40 pairs and 33 nonpaired serum samples) were from healthy non-COVID-19 adults with and without other respiratory infections. Data points are P-N values, and horizontal lines indicate mean and mean + standard deviation (SD). (A) The absorbance values of controls and COVID-19 specimens are shown in IgM ELISAs with RBD-His, S-His, and N-His as antigens. Horizonal lines indicate mean absorbance values. The cutoff values for RBD, S, and N are 0.060, 0.101, and 0.100, respectively. (B) The absorbance values of controls and COVID-19 specimens are shown in IgG ELISAs with RBD-His, S-His, and N-His as antigens. Horizontal lines indicate mean absorbance values. The cutoff values for RBD, S, and N are 0.114, 0.090, and 0.177, respectively. Note the difference in scale for the IgM-N assay.

FIG 3

Longitudinal analysis of SARS-CoV-2-specific IgM and IgG antibodies in COVID-19 patients. Data points are IgM and IgG P-N values for serial specimens at 1, 3, and 6 months PSO from 14 COVID-19 PCR⁺ patients. The three serum samples from each patient were run at the same time. (A) The absorbance values of COVID-19 specimens (n = 14) collected at 1, 3, and 6 months PSO are shown in IgM ELISAs using RBD, S, and N antigens. The cutoff values for RBD, S, and N are 0.060, 0.101, and 0.100, respectively. (B) The absorbance values of COVID-19 specimens (n = 14) collected at 1, 3, and 6 months PSO are shown in IgG ELISAs using RBD, S, and N antigens; ns, not significant. The cutoff values for RBD, S, and N are 0.114, 0.090, and 0.177, respectively. Note the difference in scale for the IgM-N assay.