See below for answers to frequently asked questions about the use of antibody testing for SARS-CoV-2 within the COVID-19 pandemic.
While antibody tests aren’t recommended as a primary diagnostic tool for COVID-19, serology can play an important role in aiding in the diagnosis of some cases.1 It can be helpful when molecular tests are negative and COVID-19 is highly suspected, or if the patient did not have access to molecular testing early in the course of their disease.1,2 Antibody tests are useful for detecting a previous infection two weeks or more after symptom onset.1
Molecular tests are used for the initial diagnosis of SARS-CoV-2 and function by detecting the viral genome. In contrast, antibody testing measures the body’s immune response to the virus, specifically the production of antibodies that recognize specific SARS-CoV-2 proteins.2
Not necessarily. More work is needed to understand what having antibodies to SARS-CoV-2 means in regard to immunity. However, serological tests monitoring SARS-CoV-2 are essential for better understanding immune response and potential immunity in the future.2
Specificity in this case refers to the ability to accurately report a negative result for individuals who did not have COVID-19, and is often determined using patient samples collected prior to the COVID-19 pandemic. For SARS-CoV-2, it is very important that serology tests have high specificity to minimize the number of false positives.1 In areas where the prevalence of SARS-CoV-2 infection is low, poor specificity can result in greatly overestimating the number of individuals who truly had SARS-CoV-2.1,3
SARS-CoV-2 is one of seven known coronaviruses that can infect humans.4 Highly specific tests are better able to identify only antibodies to SARS-CoV-2, instead of other coronaviruses.1,3 One factor that influences specificity is the similarity of certain antigens between other viruses. For example, the RBD domain of the SARS-CoV-2 Spike protein compared to the SARS-CoV-2 nucleocapsid protein shows less similarity to the corresponding proteins in other coronaviruses.5 Therefore, tests that target the RBD may offer improved differentiation and higher specificity.5
Sensitivity generally refers to the ability to detect antibodies to SARS-CoV-2 in individuals confirmed to have the virus through molecular testing. High sensitivity suggests a higher probability that a negative result is a true negative.
Factors such as severity of disease, time since symptom onset, and isotypes being tested can all affect the sensitivity of tests used to detect antibodies against SARS-CoV-2.1
Total antibody tests can detect IgG, IgA, and IgM, thus providing a comprehensive view into the body’s immune response to SARS-CoV-2.1 If a patient fails to produce one isotype, either because they fail to seroconvert, or because they have an immunoglobulin deficiency, the total antibody test can detect the other isotypes if they are present.
In contrast, an IgG-only test specifically detects the IgG antibody isotypes. IgG is a longer lasting antibody that plays a role in long-term immunity (although long-term immunity to COVID-19 has yet to be established).1, 3
IgA and IgM are two other isotypes that may play an important role in combatting a SARS-CoV-2 infection. IgA is the predominant isotype in mucosal regions and is believed to have a neutralizing role.6 Because IgM has been shown to both peak and wane within one month post-symptom onset, it can be an indicator of a more recent SARS-CoV-2 infection.1, 7
A total antibody test is advantageous for getting an overall view of an individual’s antibody response to SARS-CoV-2, which can be valuable for epidemiological studies or for complementing molecular diagnostics for individuals in the later course of their illness.1
Compared to tests that detect antibodies to the nucleocapsid protein, RBD-based tests may show less cross-reactivity to other coronaviruses and therefore better specificity.5 The RBD is believed to be the primary epitope for neutralizing antibodies8-11, and certain applications, such as screening convalescent plasma or monitoring vaccine efficacy, may rely on detecting antibodies that are more likely to be neutralizing.1 The S protein is also a major vaccine target.12