https://www.bmj.com/content/370/bmj.m3699
Quote:
The polymerase chain reaction (PCR) swab test is useful (but not perfect) for detecting SARS-CoV-2 virus RNA in symptomatic patients.1 However, problems arise using the test for purposes that disregard symptoms or time of infectionnamely, case finding, mass screening, and disease surveillance.
This is because PCR is not a test of infectiousness. Rather, the test detects trace amounts of viral genome sequence, which may be either live transmissible virus or irrelevant RNA fragments from previous infection.2 When people with symptoms or who have been recently exposed receive a positive PCR result they will probably be infectious. But a positive result in someone without symptoms or known recent exposure may be from live or dead virus, and so does not determine whether the person is infectious and able to transmit the virus to others.
Quote:
If PCR is used to identify cases through mass testing of healthy people, it will deliver positive results in individuals with previous resolved infections, new infections, and potential re-infections, as well as false positives in people genuinely not harbouring the virus (around 0.8% of all tests performed8). Identifying the truly infectiouswho must isolateis not straightforward, even with a clinical history. For example, between 4% and 41% of cases are asymptomatic, with a risk of transmission roughly half that of symptomatic cases,9 but a positive test in those with no history of symptoms could indicate either current infection or previously resolved asymptomatic infection.
Real concern exists that many people who are not infectious (and not likely to become infectious) will receive positive test results, and together with their contacts, will be forced to isolate unnecessarily. In the context of mass surveillance, this could be a majority of those who test positive. Using PCR for population screening - even with a lower maximum Ct value cut off - is not epidemiologically sound. The balance of costs and harms against the potential benefits has not been evaluated.
