Are Rapid Immunodiagnostic Tests for COVID-19 Ready for Prime Time?

The success of treatment, quarantine and, ultimately, return to normalcy, all hinge on the development and dissemination of tests capable of not just accurate but rapid detection of COVID-19 coronavirus. Among the most promising solutions are point-of-care (POC) molecular-based immunodiagnostic tests that detect SARS-CoV-2 antibodies. On March 21, the Cepheid Xpert Xpress SARS-CoV-2 test became the first POC COVID-19 detection assay to receive Emergency Use Authorization (EUA) from the US Food and Drug Administration. More POC test approvals followed in rapid order, including assays from Abbott, Mesa Biotech and the combination of Becton Dickinson and BioGX. And dozens more are in the pipeline and likely to gain EUA in the coming months, if not weeks. Hip-hip-hooray! Right? Well, maybe not so much, at least to some critics. The cloud to the silver lining is that POC COVID-19 antibody testing is unproven. So, even as the FDA admits them into the US market, the World Health Organization (WHO) says they shouldn’t be used for clinical decision making. The Diagnostic Challenge Most of the current COVID-19 tests, including the US Centers for Disease Control and Prevention (CDC) that received the initial EUA from the FDA, are based on reverse transcriptase polymerase chain reaction (RT-PCR) detecting genetic material from the SARS-CoV-2 virus from respiratory samples taken with a throat or nasal swab. The way it works: SARS-CoV-2 is an RNA virus, which must be converted to DNA at the laboratory using the RT enzyme. Laboratories then add specific sequences of DNA (primers) capable of recognizing complementary virus sequences, so that another enzyme—typically a modified form of Taq polymerase—can make a copy of a short length of viral DNA. The process is repeated over 20 to 30 cycles to expand the amount of DNA that can be detected. The good news about RT-PCR detection is its accuracy, with both sensitivity and specificity rates of 90 percent and above. The bad news about the method is that it takes a long time and usually must be carried out at a testing laboratory away from the site of care. The hours and days patients must await their test results is not only agonizing but ill-suited to the logistics and dynamics of social distancing which calls for rapid identification of those who are healthy and those who need to be in isolation. POC Immunodiagnostic Tests The POC immunodiagnostic tests utilize the same basic methodology as the laboratory assays. But several of the steps are automated and performed at an accelerated pace on small, portable platforms that can be used right at the doctor’s office, hospital or other care setting—and even a patient’s home—without being transported to an off-site laboratory. As a result, test results can be generated in under an hour as the patient waits. But there is also a fly in the ointment, namely, test accuracy. The other key difference between RT-PCR and rapid immunodiagnostic testing is that the latter detects COVID-19 indirectly. Rather than detecting viral genetic material, rapid tests target the patient’s immune response by detecting antibodies against the virus or virus antigens. The problem is that antibodies develop several weeks after an infection. This lag between time of testing and time of antibody development can result in false negatives, especially for asymptomatic cases or patients in the earliest stage of the disease. And because antibodies can remain in the body years after the infection goes away, people who do not have the virus may still test positive. The WHO Response This tradeoff between POC immunodiagnostic testing speed and lack of accuracy is forcing policy makers and public health officials around the world to make some difficult decisions. As noted above, the US seems to be all in on rapid tests, at least for as long as the COVID-19 pandemic lasts. But the WHO is not so sure. On April 8, the organization issued a scientific briefing saying that POC immunodiagnostic SARS-CoV-2 tests should not be used for clinical decision-making “until evidence supporting use for specific indications is available.” Before such tests can be recommended, “they must be validated in the appropriate populations and settings,” according to the WHO. “Inadequate tests may miss patients with active infection or falsely categorize patients as having the disease when they do not, further hampering disease control efforts.” Molecular RT-PCR testing “of respiratory tract samples is the recommended method for the identification and laboratory confirmation of COVID-19 cases,” according to the WHO. But the organization did endorse use of POC immunodiagnostic tests for epidemiological research. “Tests to detect antibody responses to COVID-19 in the population will be critical to support the development of vaccines, and to add to our understanding of the extent of infection among people who are not identified through active case finding and surveillance efforts, the attack rate in the population, and the infection fatality rate,” according to the WHO. Antigen Detection Tests for Triage However, the WHO does see a limited treatment role for another kind of POC testing, i.e., rapid diagnostic tests (RDTs) that detect the presence of viral proteins, or antigens, expressed in a sample from the respiratory tract by the SARS-CoV-2 virus. “If any of the antigen detection tests that are under development or commercialized demonstrate adequate performance, they could potentially be used as triage tests to rapidly identify patients who are very likely to have COVID-19, reducing or eliminating the need for expensive molecular confirmatory testing,” the WHO said. The caveat: Based on experience from their use for other respiratory diseases such as influenza, how well antigen-based RDTs work may vary from 34 percent to 80 percent, based on factors such as:

• The time from onset of illness;
• The concentration of virus in the specimen;
• The quality of the specimen collected;
• How the specimen is processed; and
• The precise formulation of the reagents in the test kit.

“Based on this information, half or more of COVID-19 infected patients might be missed by such tests, depending on the group of patients tested,” the WHO added. “These assumptions urgently require further study to understand whether they are accurate.” In addition, the WHO warned of false positives that may result if the antibodies on the test strip also recognize antigens of viruses other than COVID-19, such as from human coronaviruses that cause the common cold. Takeaway At a time where diagnostic speed is of the essence, testing laboratories are at the limits of their capacity and reagents are in short supply, these POC tests detecting SARS-CoV-2 antibody seem like the perfect solution. And test makers have responded by adapting their previous pathogen detection assays to COVID-19. But, as the WHO reminded us, there is also a major risk in relying on these tests unless and until they are proven accurate enough for use in clinical decision-making.