Is Covid Testing Too Sensitive?
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Apoorva Mandavilli @apoorva_nyc · Aug 29 NEW: All these months into the pandemic, we may have been testing the wrong way. Data from some state labs suggest up to 90% (!!) of people who get a positive result are no longer contagious and don’t need to isolate. |
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Apoorva Mandavilli @apoorva_nyc · Aug 29 It turns out that the PCR, that old reliable workhorse, is both too slow and too sensitive for what we need. And it all hinges on a metric called the “cycle threshold.” Here’s a handy explanation of what the CT value is and why it matters. 2/x |
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Apoorva Mandavilli @apoorva_nyc · Aug 29 Essentially, the PCR test amplifies genetic matter from the virus in cycles; the fewer cycles required, the greater the amount of virus, or viral load, in the sample. The greater the viral load, the more likely the patient is to be contagious. 3/x |
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Apoorva Mandavilli @apoorva_nyc · Aug 29 Here’s where it gets truly shocking. Most PCR machines at state labs are set to a cutoff of 40 or 37, meaning it takes that many cycles to get a signal. But acc to most virologists, at that level, you are amplifying so little virus that it might almost be an artifact. 4/x |
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Apoorva Mandavilli @apoorva_nyc · Aug 29 If you adjust that down to a more reasonable CT threshold of 30, anywhere form 40%-90% of state lab results are *no longer positive.* The rest are well past the point of contagiousness. 5/x |
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Apoorva Mandavilli @apoorva_nyc · Aug 29 Think about that for a minute. Think of the thousands of people being asked to isolate, refrain from work and submit to contact tracing. But also, think of the needless bottlenecks, and all the people who aren’t getting tested and isolated *when* they need to be. 6/x |
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Apoorva Mandavilli @apoorva_nyc · Aug 29 Imagine a neighborhood on fire. Here, the firefighters have defined even dying embers as a “fire” and are so busy putting those out that they are missing entire homes that are burning down and setting others ablaze. 7/x |
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Apoorva Mandavilli @apoorva_nyc · Aug 29 Let’s be very clear: this is not an argument for testing less. It’s an argument for testing MORE and testing more OFTEN. With rapid, less sensitive tests that tell you what you actually need to know from a public health perspective: whether you are contagious. 8/x |
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Apoorva Mandavilli @apoorva_nyc · Aug 29 Apart from the mind-blowing public health implications of these numbers, we are also losing valuable scientific intel about the virus and disease trajectory by not paying attention to CT values and delivering just yes-no answers (per FDA authorizations). 9/x |
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Apoorva Mandavilli @apoorva_nyc · Aug 29 Massive thanks to @michaelmina_lab who alerted me to this, and to @JumoDr @angie_rasmussen @ashishkjha @scottjbecker and many others, incl at state labs. --/10 |