The UK vaccination rate is impressive but little attention is paid to the infection rate which is crucial if the virus is to be defeated. Testing has increased since last year with more laboratory capacity and wider use of self-administered lateral flow kits (LFD). Any increase in testing overall will inevitably contribute to a higher level of infection detected, but the combining of test results from two types of test can also contribute to recording higher infection. This would not matter if the two tests acted in the same way on the swab sample, but they don’t so should the results be combined?
Just to recap: the PCR test requires laboratory analysis of the genetic material of a swab sample, it is universally accepted as a reliable test for the virus, a positive reading indicates infection. The lateral flow test (LFD) by contrast only identifies a protein antigen on the virus surface; a positive reading indicates the antigen is present. This is currently taken to mean the person is infectious, but it was not always so, antigen testing was last year presented as evidence of having built some immunity. This change in interpretation is significant: if positive LFD results are now recorded as cases of infection then this will contribute to a higher infection rate being recorded.
Following trials in Liverpool and universities last year the LFD tests offered an opportunity to identify people who were infectious yet asymptomatic. LFD tests were seen as a route to community testing and in December were licensed for self-administration. They can produce a result within 15 minutes and do not require laboratory analysis. Despite some reservations about accuracy and bias towards negative results, they are now considered the first line in tracking spread of the virus across the population. Positive LFD test results are referred for secondary PCR test verification, but still initially logged as a case of infection based on the LFD result.
What is the purpose of antigen testing and how should it be interpreted? During the first lock down last March the government bought millions of antigen tests (finger prick blood sampling) for self-administration by the public, but these were found to be inaccurate and the test programme aborted. Antigen tests last March were presented to the public as a way to detect if you had caught the virus without knowing, and if your immune system had subsequently created some antibody defence should it return. A positive result showed you had some protection, not you were infectious.
Since trialled through community testing in Liverpool the LFD test has been used to detect the ‘unconsciously infectious’ with the intention of removing them from circulation via isolation. In itself a sound public health policy, but it distorts what a positive LFD test result indicates. Are we now recording positive LFD test results as active infectious individuals, when we should be recording them as protected individuals who exhibit some level of immunity? Moreover are these positive tests being lumped with PCR positive results to give a combined infection rate? Should the results of a lab analysed genetic sample be combined with a quick protein antigen sample to give one single infection rate metric? I think the public deserve more detail on testing and less on vaccine rollout.
