PCR (polymerase chain reaction)
PCR tests detect the genetic information of the virus, the RNA, which is only present if someone is actively infected. The tests directly screen for the presence of the viral RNA, rather than the presence of the body’s immune response, or antibodies. By detecting the viral RNA, which may be present before symptoms show up, the tests can tell whether or not someone has the virus early on.
By scaling PCR testing to test vast groups in a population, doctors can initiate treatment sooner and public health officials can quarantine people and perform contact tracing. Swab PCR tests are theoretically 100% accurate. Current rules say that swab PCR swab tests should be used for anyone who has symptoms or a known exposure to COVID-19. Swab PCR tests can be used in hospitals and clinics that have laboratories. However, PCR tests have several limitations that will prevent testing 330 million Americans.
Swab PCR tests are the most accurate tests for current infection, and there is a need to drastically scale up production of PCR swab test kits. But they are expensive, labor-intensive, somewhat prone to sample-collection error, and require lab equipment. Faulty collection techniques may retrieve only mucous or debris but not virus particles. Improper swab collection can lead to false negative rates as high as 50%.ix
There are currently only a few hundred sites that are capable of processing swab PCR tests across the country.x In addition to the wide swaths of the country that don’t have access to PCR swab tests (see diagram), only certain laboratories have the expensive machines and staff trained to operate those machines. These hurdles make PCR swabs more expensive and limit testing available to “healthcare deserts” in many rural and densely populated poor urban areas. Populations in nursing homes, Native American reservations, prisons, homeless shelters, and communities of color will continue to lack easy access to swab PCR tests. COVID-19 does not distinguish between rich and poor. If testing doesn’t reach everyone, that affects everyone across all bands of society, and economically disadvantaged populations will serve as reservoirs of disease.
Swab PCR swab tests, the “gold standard” for diagnosis of many disease types, has been shown to have problems when used for COVID-19. The development of CDC’s early PCR swab tests encountered problems, such as cross contamination of certain lab components that led the test to give unusually high rates of false positives against a negative control, according to the CDC.
Furthermore, swabs may miss early infection due to the disease’s long incubation period. Asymptomatic patients may not shed enough of the virus for the swab to pick up, and nasal swabs are frequently used incorrectly: they are uncomfortable for patients. One study using preliminary Chinese data found a false negative rate of 30%. Some Chinese public health experts estimate PCR swab accuracy in Wuhan was as low as 50%. As health systems are stressed, accuracy rates for PCR decline. Meanwhile, the rollout of other tests was delayed by the FDA.
More portable PCR swab tests will greatly help in the fight. A handheld PCR test was approved for use in Canada, increasing the portability and accessibility of such tests, but is not available in the US yet. Recently, a new fast and mobile PCR test by Abbott Labs has received preliminary FDA approval and boasts high accuracy in laboratory settings. However, the preliminary studies were conducted on only 60 patient samples, and all samples were “contrived clinical nasopharyngeal swab specimens,” artificially spiked with virus, instead of being taken directly from sick people, severely limiting the ability to generalize this lab data into the real world. While the test may be a great tool, Abbott has said that it can make only 1.5 million of these per month. In addition, the Abbott test faces the same problems as other PCR tests when used in the real world, especially in cases where the virus is present in lower concentration