Serology
Serologic tests that identify antibodies (such as IgA, IgM, and IgG) to SARS–CoV-2 from clinical specimens (such as blood or saliva), such as enzyme-linked immunosorbent assays, may be less complex than molecular tests and have the potential to be used for diagnosis in certain situations (46). However, their utility for diagnosing acute infections is probably limited around the time of symptom onset, when viral shedding and transmission risk seem to be highest (32). Antibody responses to infection take days to weeks to be reliably detectable (46). Negative results would not exclude SARS–CoV-2 infection, particularly among those with recent exposure to the virus. Cross-reactivity of antibody to non–SARS–CoV-2 coronavirus proteins is also a potential problem, whereby positive results may be the result of past or present infection with other human coronaviruses (47). Serologic assays might be more relevant in scenarios in which patients present to medical care with late complications of disease, when RT-PCR may be falsely negative, because viral shedding drops over time (48).
The development of serologic assays that accurately assess prior infection and immunity to SARS–CoV-2 will be essential for epidemiologic studies, ongoing surveillance, vaccine studies, and potentially for risk assessment of health care workers. Immunoassays are already on the market in some countries, but their diagnostic accuracy and optimal use remain undefined.