When someone gets infected with a virus, like SAR-CoV-2, their body mounts an immune response by developing antibodies against this virus. These antibodies attack the virus and clear the virus from the system. If someone contracts COVID-19 but shows no symptoms, Antibody tests, also known as serology tests, tell us if an individual was infected in the past and has an immune response.
Serology testing does not detect the presence of the virus, but rather detects the antibodies that are, or were, produced by the body in fighting the disease.
Antibodies are proteins produced by the immune system in response to an infection and are specific to that particular infection. They are found in the liquid part of blood called serum or plasma.
Immunoglobulin M, or IgM, is the first type of antibody produced in response to an infection and is detectable 4 to 7 days after an infection starts. IgM antibodies are short lived and their existence signals a new infection is present.
Immunoglobulin G, or IgG, antibodies are produced 7 to 14 days after infection. These antibodies can be detectable for weeks, months and even years, depending upon the antigen and the individual. IgG antibodies are usually sustained for a longer time and may play a role in lasting immunity. However, since we don’t know a lot about SARS-CoV-2, as it’s a new virus, we don’t know how immunity against this virus works.
Total assays, which are a combined IgG-IgM tests, cannot distinguish between early IgM and late IgG antibody responses. As a result, total assays can’t reveal whether a person has the IgG antibodies that are needed for long term immunity, or if they are currently infected. Total Antibody test is intended for use as an aid in identifying individuals with an adaptive immune response to SARS-CoV-2, indicating recent or prior infection. IgG and IgM antibodies to SARS-CoV-2 are generally detectable in blood several days after initial infection, although the duration of time for which antibodies are present post infection is not well characterized.
Research has shown that severe and critical cases had higher IgM levels than mild cases, whereas the IgG level in critical cases was lower than those in both mild and severe cases. This might be because of the high disease activity and/or a compromised immune response in critical cases. The IgM antibody levels were slightly higher in deceased patients than recovered patients, but IgG levels in these groups did not significantly differ. A longitudinal detection of antibodies revealed that IgM levels decreased rapidly in recovered patients, whereas in deceased cases, either IgM levels remained high or both IgM and IgG were undetectable during the disease course.
The right type of serology test could help us understand immunity against COVID19 at an individual and a population level. This can be a key tool to help us develop plans to reopen society.
How to evaluate the diagnostic performance of serological IG g antibody response in COVID-patients
Many studies have demonstrated limited clinical sensitivity of the IgG assay, particularly in the earlier stages of COVID-19 illness. IgG testing is not suitable for laboratory diagnosis in acute disease, but is considered for retrospective testing for epidemiological purposes. Although high technical specificity was demonstrated, clinicians should be aware of the limitations of IgG testing in interpreting the results
Appropriate thresholds for sensitivity and specificity of an antibody test depend on its purpose and must be considered prior to implementation. For diagnosis in symptomatic patients, high sensitivity is required (generally ≥90%). However, if antibody tests were deployed as an individual-level approach to inform release from social isolation and return to normal activities, high specificity would be essential, as false-positive results would return non-immune individuals to risk of exposure. Various studies have revealed a sensitivity of 93.1% and specificities 99.2% for IgG test. This indicates that the test is suitable for assessing previous virus exposure.
Testing of IgG after 14 days from the onset of illness may be considered for retrospective identification of patients. However, it should be noted that the interpretation of IgG testing may change as more patients develop baseline antibodies against the virus. SARS-CoV-2 IgG testing would also be instrumental in vaccine development, clinical trials, and epidemiological studies.