COVID-19, the novel coronavirus disease caused by SARS-CoV-2, can be detected in patients using serological testing. COVID-19 has been shown to follow a standard seroconversion curve. This means that IgM antibodies are produced approximately 12 days after symptom onset by B cell derived plasma cells. These IgM antibodies are later outnumbered by the more robust IgG type antibodies, which mark the generation of a maturing or secondary immune response. The process of switching from primarily IgM to primarily IgG antibodies in the serum of the patient is known as isotype or class switching.

SARS-CoV-2 infections have been shown to begin isotype switching approximately 15 days after the initial presentation of symptoms. At this point, IgM and IgG antibodies are both present in the serum.

Our two newest COVID-19 ELISA kits, for IgG and IgM detection, utilise indirect-ELISA methods to qualitatively detect SARS-CoV-2 virions in plasma or serum samples. In particular, these two kits test for the presence of the nucleocapsid ‘N’ protein, which is an integral component of the SARS-CoV-2 virus.  

The kit contains a dismountable micro-ELISA plate that is pre coated with the purified N protein antigen. Once samples are added to the wells, any antibodies from within the serum or plasma sample will bind with the N protein that is fixed to the plate. After the plate has been completely washed, the horseradish peroxidase conjugated mouse-anti-human IgG is added to the mix. This forms the secondary antibody complex that will ultimately be measured in the detection step.

Finally, after the free components are washed away, the substrate solution is added to each well on the plate. Only wells with the human antibody/N protein/mouse antibody complex will appear blue. The enzyme/substrate reaction is terminated by the addition of stop solution, which will turn the colour yellow. Be sure to follow the best practices for ELISA kit usage to ensure that the results are clear and reproducible.

Once complete, the data garnered from the ELISA kit can be analysed. There are a number of ways to optimise your kits for more robust accuracy and reproducibility, and it is strongly recommended to follow these steps in order to ensure maximised kit performance. Similarly, we’ve compiled over 100 ELISA troubleshooting tips for any issues that may arise. Currently, both the IgM and IgG kits are approved for research use only.