Antibody testing, also called serological testing, can be used to determine whether an individual is currently infected with COVID-19 or has previously been infected with COVID-19 and recovered. Unlike the molecular testing (PCR tests), which detects the presence of genetic material from the virus from a nasopharyngeal or oropharyngeal swab, these antibody tests identify exposure to SARS-CoV-2 (the virus which causes COVID-19) by looking for antibodies generated by the immune response. Most SARS-CoV-2 antibody tests detect IgM and/or IgG. IgM is the first antibody the body builds when fighting a new infection, and may indicate you are still infected or recently recovered. IgG, on the other hand, takes 7 to 10 days to develop, and indicate you have previously been infected and recovered.
Antibody testing detects the body’s immune response to the infection, rather than detecting the virus itself. As such, there are limitations to the effectiveness of antibody testing in diagnosing COVID-19. At the start of an infection, especially in the first day or two, the body may not have developed sufficient IgM antibodies to be detectable in peripheral blood. Individuals who exhibit very mild to no symptoms, including asymptomatic carriers, may not mount a sufficient immune response to generate detectable levels of antibodies. Furthermore, some individuals who are immunocompromised may not build enough antibodies and therefore lead to false negatives.
Despite these limitations, antibody testing still plays a role in the COVID-19 pandemic. Antibody testing can be useful in screening individuals without symptoms. Many antibody tests are rapid tests, which involves only a fingerstick, and yields results in 10 to 20 minutes. Due to supply limitations and laboratory limitations to the number of molecular tests that can be performed, it would be extremely difficult (if not impossible) to test everyone with the nasopharyngeal or oropharyngeal swab. In addition, the molecular tests only capture whether or not the individual is infected at that current time – the individual can become infected the next day.
Another reason for antibody testing, and perhaps one of the most important reasons, is its use in research and epidemiology – in helping to understand the impact of the virus. By tracking exposure rates to the virus, safety protocols and interventions can be implemented to ensure public health and safety. By tracking an individual’s antibody titers, we can determine how long these antibodies remain in the body and whether or not an individual can build an immunity to the virus. Furthermore, by identifying individuals who have recovered and developed IgG antibodies, we can identify potential plasma donors who may be beneficial in treating other COVID-19 patients.
Molecular and serological testing are different things, and both play crucial roles in battling this pandemic.
Christine Lau is a physician.
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