2020/11/19
The Lancet published an article in September covering the suitability of the PCR test for Covid-19 diagnostic purposes.
Obviously it is a technical document but the case it makes is in principle easy to understand - the PCR "gold standard" test has value when applied in a clinical setting for a patient who is exhibiting symptoms and was designed accordingly. But . . .
"diagnostic or operational performance of swab tests in the real world might differ substantially from the analytical sensitivity and specificity" (I interpret "in the real world" to be equivalent to "outside the clinical setting").
The author goes on to make the point that whilst the test was designed initially to avoid false negative results, since it was applied initially in healthcare settings where the primary risk was perceived to be that of missing infections that might spread amongst healthcare workers and thus incapacitate the system, this bias is less appropriate when the test is applied outside the clinical setting as asymptomatic people are being tested in the general population.
In other words the positive tests that will result will not reflect the true incidence of active infection in the population at large, as a bias toward reducing false negative results will inevitably tend toward increasing the false positives.
"When low pretest probability exists, positive results should be interpreted with caution and a second specimen tested for confirmation. Notably, current policies in the UK and globally do not include special provisions for those who test positive despite being asymptomatic and having laboratory confirmed COVID-19 in the past"
This informative and accessible article goes on to list the potential consequences of false-positive COVID-19 swab test results at the individual and societal levels, and to discuss further aspects of the technicalities pertaining to the test and to its application.
