LFT vs PCR17 Feb 2021 10:36
In today’s Lancet,
A huge endorsement of accurate LFT testing :
Clarifying the evidence on SARS-CoV-2 antigen rapid tests in public health responses to COVID-19.
Testing for SARS-CoV-2 is central to COVID-19 manage- ment and has relied on quantitative reverse transcriptase Polymerase Chain Reaction (PCR) technology. PCR seeks the genetic code of the virus from nose or throat swabs and amplifies it over 30–40 cycles, doubling each cycle, enabling even miniscule, potentially single, copies to be detected. PCR is thus a powerful clinical test, specifically when a patient is, or was recently, infected with SARS- CoV-2. Fragments of RNA can linger for weeks after infectious virus has been cleared,6 often in people without symptoms or known exposures.7
However, for public health measures, another approach is needed. Testing to help slow the spread of SARS-CoV-2 asks not whether someone has RNA in their nose from earlier infection, but whether they are infectious today. It is a net loss to the health, social, and economic wellbeing of communities if post-infectious individuals test positive and isolate for 10 days. In our view, current PCR testing is therefore not the appropriate gold-standard for evaluating a SARS-CoV-2 public health test.
The short window of transmissibility contrasts with a median 22–33 days of PCR positivity (longer with severe infections and somewhat shorter among asymptomatic individuals).
This suggests that 50–75% of the time an individual is PCR
positive, they are likely to be post-infectious.
At this point, a priori one should expect a
public health test that is highly sensitive for detecting infectious virus to show low overall sensitivity relative to PCR in people without symptoms or known exposures.
In our view, the Birmingham study showed that PCR- positive asymptomatic students at a time of falling COVID-19 incidence had low viral loads compared with symptomatic members of the public attending testing centres and that LFT was working as expected.
We wholeheartedly support healthy scientific debate to inform policies promptly. The COVID-19 road ahead looks challenging, therefore, we need big, bold actions across science and society, such as the Liverpool community testing pilot. The prompt evidence from such pilots can inform policies and help maintain public confidence in the public health responses needed to
navigate this pandemic’s onward path.
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