The Economist April 4th 2020 pp64-65 Science & Technology “Testing testimony” “The coronavirus pandemic” “How an antibody test for the novel coronavirus should- and should not-be used”.
Background
Testing for COVID-19
Due to limited availability of testing and concerns about overwhelming healthcare facilities, testing for active COVID-19 tested has been largely restricted to those with the appropriate signs and symptoms. Testing for active COVID-19 are determined using a nasopharyngeal specimen from which viral genes are eluted, converted from RNA to DNA for DNA testing by PCR. Specimens that are correctly collected from symptomatic individuals should show the presence of COVID-19. False negatives, as much as 30%, can happen if swab sampling is insufficient.
As with most infectious disease, the immune system responds by producing antibodies that help eliminate the infection first by a class of antibodies known as IgM and later IgG. These tests can help determine if an individual has been exposed and presumably had some form of COVID-19 disease and are more likely immune to future COVID-19 infections. Identifying these antibodies will be helpful for screening healthcare workers and other public-facing workers for return to work. Testing a large sample of various populations, by public health organizations, will render an accurate picture of the overall cumulative incidence of COVID-19. This information will help us understand what fraction of the population presumably has immunity often referred to as “herd immunity”. Higher herd immunity, the less the future likelihood of disease spreading as rapidly. At this point, an improper use of testing would be spending public bandwidth for testing and public money for the curious or “worried well”. Eventually, and until a vaccine is available, those wishing to spend their own funds might have legitimate personal and professional reasons for knowing if they had a form of COVID-19-not wanting to infect others and wanting to resume travel without concern for developing serious COVID-19 away from home. With current methods, testing would not be insightful if serum was collected well after having presumptive COVID-19.
The immune response is so effective, that transferring immunity is possible by infusing appropriately processed and matched blood serum from recently recovered patients. Patients proven to have had clinical disease and viremia-virus detected in blood are the source of what is called convalescent blood plasma. The level of virus-fighting antibodies was measured in Chinese testing using the ability of the plasma to stop the growth of virus in cultured cells infected with COVID-19. This very analytically sensitive and specific testing, carried out in BSL-4* laboratories, identified individuals with very high concentrations of anti-COVID-19 antibody. These plasmas, after processing and matching, successfully rescued severely ill patients as evidenced by laboratory testing, oxygen-saturation levels and improved lung CT scan etc. This type of treatment is called adaptive immunity and has been used throughout modern medical history. Reference PNAS.
Tests approved by the FDA are just emerging. It is unclear, in these early days, what the analytical sensitivity (the ability to detect low levels of antibody) and analytical specificity (the ability to only detect SARS-COV-2 [AKA COVID-19]) of testing kits FDA approved or not. Further, we don’t yet have enough information on the clinical sensitivity (ability to detect all patients with previous disease) and clinical specificity (ability to detect only patients with SARS-CoV-2). If the goal is being sure that positive tests are reliable then a lower clinical sensitivity and higher clinical specificity is warranted. See table below explaining test interpretation.
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