The Economist November 14th, 2020 pp71-72 |Science & technology| “Fast diagnosis for covid-19” “Test Match” “Cheap, rapid tests for SARS-COV-2 are here. Will they be the stopgap needed before a vaccine is deployed?”
Current COVID-19 tests are almost exclusively performed in commercial, clinic or hospital laboratories on swabs taken from the nose or oropharynx using what is called Reverse Transcriptase Real Time Polymerase Chain Reaction (RTPCR). This technology uses RNA from COVID-19, AKA SARS-COV-2, converts it to DNA and then makes many identical copies of that DNA to levels that are easily and accurately detected. These tests require specialized equipment, trained laboratory workers, specialized testing laboratories and many operating and quality procedures designed to safely and reliably perform such testing of swabs delivered from the patient bedside to the laboratory. RTPCR for COVID-19 in this setting is the most analytically and clinically sensitive, detects the smallest amounts of COVID-19 and only when truly present, and most analytically and clinically specific, detects only COVID-19 and not other viruses or "background noise."
As it turns out, having said all this, no laboratory test is 100% perfect. With COVID-19 RTPCR there are set-points that once exceeded are resulted as positive. While the setpoints can be chosen, after analytical and clinical validation, to be most reliable in discriminating between those with COVID-19 from those without COVID-19, each individual test measurement still has some variance. If you measured the same swab solution many times you will have some identical and some non-identical results. Some of these results, if the patient is without COVID-19 or just starting to develop or ending COVID-19, will fall on the negative side of the setpoint and some will fall on the positive side of the set point. Especially in the case of individuals clearly without exposure to COVID-19 and clearly not infected with COVID-19 those results falling on the positive side of the set point are false positives. Conversely, if the patient has signs and symptoms of COVID-19 then it's likely that all of results will be easily beyond the setpoint. For this reason, all diagnostic testing of this type, is most accurate in evaluating patients with a high likelihood of disease. With that as a preface, simpler tests that can even be performed at home by people without training or specialized equipment are becoming available and for a cost as low as $1 eventually. These simpler tests will be very fast and can be helpful in identifying individuals who are developing or have COVID-19. If tested positve then these patients can quarantine and contact trace immediately. As described above, COVID-19 tests are most effectively used to evaluate patients exposed to COVID-19 or clearly have symptoms of COVID-19.
These simple tests using either swabs or saliva are being approved and will be widely available in the coming months. These simple tests use a different technology that detects COVID-19 unique molecules sometimes called proteins, peptides or antigens rather than COVID-19 RNA. The readout is similar to a home pregnancy test, see figure. In the biology of COVID-19 any infection will contain COVID-19 RNA and COVID-19 antigens but the technology for measuring antigens is less able to detect low levels. This means that antigen tests are even less useful, than RTPCR, in evaluating individuals without COVID-19 symptoms-there will be many false positives if the general population is screened. So far two of these COVID-19 antigen tests have FDA Emergency Use Authorization-many more are in the pipeline. The technology is not new and can be scaled up quickly and should add as many as 2.5M more tests per day. If used appropriately, in patients with COVID-19 symptoms or individuals exposed to COVID-19, these rapid tests self-performed at home, in the clinic or at work will help identify those needing to quarantine and contact trace. Having such testing will add another tool, along with masking and vaccine, to stop the spread of COVID-19.