Rethinking Covid-19 Test Sensitivity — A Strategy for Containment: (Antigen Tests!!!)

A commentary was published in New England Journal of Medicine yesterday "Rethinking Covid-19 Tests Sensitivity -- A Strategy for Containment" that makes a case that we are thinking of testing all wrong, and that PCR tests are not effective for containment and we should adapt our strategy to use faster, less expensive tests such as antigen tests. I 100% agree with this point, and have been pushing this idea in the context of testing in schools in previously blog posts here and here and here -- that fast, cheap testing at schools could be the key tool for keeping schools open -- preventing surges while avoiding unnecessary quarantines.

The key argument I constantly hear from people is that "antigen tests are less sensitivity than PCR tests so you would miss too many infections". As they explain in this commentary, antigen tests may be sensitive enough to detect cases that are currently infectious, and the extra positives from PCR may be dominated by low mRNA levels indicating extremely low viral loads and/or mRNA fragments from dead virus leftover from a previous infection, neither of which is the focus for containment strategies. And quarantining such people unnecessarily puts a strain on the economy and other societal functions.

Here is a key excerpt from their commentary that explains the reasoning:

"With Covid-19 cases accelerating or plateauing throughout much of the world, we urgently need to shift our attention from a narrow focus on the analytic sensitivity of a test (the lower limit of its ability to correctly detect small concentrations of molecules in a sample) to the more relevant measure of a testing regimen’s sensitivity to detect infections (the probability that infected persons learn they’re infected in time to be filtered out of the population and prevent spread to others). A point-of-care test that was inexpensive enough to use frequently would have a high sensitivity for detecting infections in time to act, without having to meet the benchmark analytic limit of detection (see diagram)" -- and here is the diagram they have created to illustrate this concept.

While illustrative, this shows how a test with lower analytical sensitivity may be preferable to PCR for detecting cases with infectious viral loads.

I'm excited to see this idea being promoted, and hope the clear elucidation of these points in this commentary will lead to better understanding and use of our testing resources to more effectively manage the viral spread.

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