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September 23, 2021
SARS-CoV-2 testing options that screen for active illness or past infection have expanded rapidly over the last few months. As institutions craft guidelines on optimizing polymerase chain reaction (PCR) diagnostic tests and serology antibody tests, some labs have been developing their own testing and result reporting algorithms.
Since the U.S. Food and Drug Administration’s (FDA) first emergency use authorization of antibody tests in March, developers have responded in high numbers, prompting the FDA, other federal entities, and organizations to issue comprehensive guidance on the uses for serologic testing. AACC, in its recommendations on SARS-CoV-2 antibody testing, covers the utility, performance, and limitations of these assays, and what they should and shouldn’t be used for in managing COVID-19, the illness caused by the new coronavirus. “Serology testing is complementary to molecular diagnostic testing in managing the COVID-19 pandemic. It may play an important role in assessing the prevalence of the disease and may support epidemiological efforts such as contact tracing while research into anti-viral therapies and vaccines continues,” AACC stated.
AACC stressed that antibody tests should not be used as a primary method to diagnose acute infection or exclude SARS-CoV-2 infection when a patient presents with symptoms. In other guidance, TriCore Reference Laboratories stated that serology testing should be used to indicate infection following a negative PCR test in symptomatic patients who later develop the illness, and to identify previously infected people for convalescent plasma donation.
The American Medical Association further clarifies the limitations of serology tests, including the risk for false positives. SARS-CoV-2 has low disease prevalence, which means that false positives will appear among high performing tests. “Take, for example, a community of 100 individuals with a disease prevalence of 5%. If a serological test with a specificity of 95% was used in this population, it would be expected to return 5% false positives, so 5 out of the population of 100,” AMA explained.
The quality of some serology tests has been questioned. FDA responded by setting 90% sensitivity and 95% specificity performance characteristics for approved tests and added a new section to clarify which tests should no longer be distributed. As part of an independent performance validation study with the National Institutes of Health’s National Cancer Institute, FDA also published test performance data on several antibody test kits on open.fda.gov.
Other government entities have weighed in on SARS-CoV-2 testing. The White House, in its blueprint on testing platforms, advocates for using newer technologies such as antigen and nucleic acid tests at the point-of-care to boost testing capacity, and using antibody tests to calculate community spread of SARS-CoV-2. Guidelines from the U.S. Centers for Disease Control and Prevention (CDC) offer three strategies to improve positive predictive value in SARS-CoV-2 testing: