The following are eight key points to remember from this high-quality review discussing the types of assays, use cases, and pitfalls of serologic testing for severe acute respiratory syndrome–related coronavirus-2 (SARS-CoV-2). The tables provide a wealth of information and are worth reviewing:

1. Serologic testing can: 1) aid the diagnosis of suspect cases when polymerase chain reaction (PCR) is not available or negative, 2) identify convalescent plasma donor, 3) identify immunity of essential works allowing them to resume work, 4) inform the public health response through seroepidemiologic surveillance and contact tracing, and 5) assess vaccine immunogenicity.
2. Coronavirus spike (S) and nucleocapsid (N) envelope proteins are highly immunogenic and the main antigenic targets of serologic assays. Host neutralizing antibodies (nAbs) appear to be predominantly directed at the S protein. The N protein plays crucial roles in viral replication and assembly, is highly conserved, and induces antibodies sooner than S during infection.
3. Serologic tests based on the S protein show little cross-reactivity with endemic human coronavirus antibodies. Assays based on the N protein is expected to have higher cross-reactivity.
4. Most patients seroconvert by 2 weeks after the onset of symptoms, and almost all patients have detectable antibodies by day 28. However, the relationship between seropositivity and immune protection has not yet been established for coronaviruses.
5. Sensitivity and specificity of serologic assays vary widely. There are no universal standards for reporting, and most assays have not been externally validated. Detection limits, reproducibility, and the use of different analytes (IgG, IgM, or IgA) vary widely, with unclear effect on performance.
6. Interpretation of serologic testing is challenging due to uncertainty regarding: 1) whether mild/asymptomatic cases mount a detectable humoral immune response; 2) whether the detection of antibodies correlates with protective immunity; 3) the duration of antibody response and anamnestic responses after infection; and 4) the relative importance of the humoral, cellular, and innate response.
7. Incorrectly identifying an individual as immune could provide false reassurance and place them at significant risk.
8. Interpretation of the results of serologic testing is challenging and should be based on the seroprevalence of the population, its context of use, and symptomatology. The central figure of the review provides a decision tree for interpretation.

Clinical Topics: COVID-19 Hub, Prevention

Keywords: Antibodies, Neutralizing, Contact Tracing, Coronavirus, COVID-19, Immunity, Humoral, Nucleocapsid, Public Health, Primary Prevention, SARS Virus, Seroepidemiologic Studies, Serologic Tests, severe acute respiratory syndrome coronavirus 2

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