The NY Times published a story Saturday which looked at the accuracy of the most popular coronavirus tests currently in use. What if found is that the tests are being run in such a way that they are far too sensitive. The Times estimates that up to 90 percent of the people who receive a positive test are carrying so little virus that they probably aren’t contagious. Only about 10 percent would need to consider isolating themselves.
To understand the story, you need to understand how the PCR test works. Basically, a sample is run through a machine that amplifies any DNA particles found in it. Each time you run the sample through the machine you amplify the DNA a bit more. And eventually, at some limit that is pre-determined, currently that is often 37 or 40 cycles, you stop. The result of that test, either positive or negative, is what gets sent out to people. If there’s no positive after 40 cycles, you don’t have the virus. But at 40 cycles, even if you test positive you may only have remnants of the virus that are no longer active.
And that’s the problem. People who aren’t a threat for spreading the virus are getting positive results simply because the samples are being run through the machine so many times.
This number of amplification cycles needed to find the virus, called the cycle threshold, is never included in the results sent to doctors and coronavirus patients, although it could tell them how infectious the patients are.
In three sets of testing data that include cycle thresholds, compiled by officials in Massachusetts, New York and Nevada, up to 90 percent of people testing positive carried barely any virus, a review by The Times found.
On Thursday, the United States recorded 45,604 new coronavirus cases, according to a database maintained by The Times. If the rates of contagiousness in Massachusetts and New York were to apply nationwide, then perhaps only 4,500 of those people may actually need to isolate and submit to contact tracing…
Any test with a cycle threshold above 35 is too sensitive, agreed Juliet Morrison, a virologist at the University of California, Riverside. “I’m shocked that people would think that 40 could represent a positive,” she said…
The C.D.C.’s own calculations suggest that it is extremely difficult to detect any live virus in a sample above a threshold of 33 cycles.
Epidemiologist Dr. Michael Mina from the Harvard T.H. Chan School of Public Health told the Times that a proper cutoff would be somewhere between 30-35 cycles. That would mean the amount of viral DNA in the sample would have to be 100 to 1,000 times higher to generate a positive result. In other words, you’d limit the positives to people who are really ill and who likely need to isolate themselves. Instead, we’re sending positive test results to thousands of people who probably feel fine because they are fine and are no threat to anyone.
The counterargument is that when people are first infected they may have a lower amount of virus in their symptoms. So the high cycle testing may detect some people who are just on the cusp of getting it where a low cycle test would miss them. Still that sensitivity is creating a lot of error for people on the other end who are largely healthy.
The best solution, according to Dr. Mina, would be to use quicker, cheaper and less sensitive tests. They may miss some people but if there are enough of them available, people could get re-tested a few days later. The Trump administration announced last week that it would be purchasing 150 million of the rapid tests:
Today, as part of an historic initiative led by the U.S. Department of Health and Human Services (HHS) and the Department of and Defense (DOD), the Administration awarded a contract for $760 million to Abbott for delivery of 150 million rapid, Abbott BinaxNOW COVID-19 Ag Card Point of Care (POC) SARS-CoV-2 diagnostic tests to expand strategic, evidence-based testing in the United States. Testing will be potentially deployed to schools and to assist with serving other special needs populations.
So this sounds like a move in the right direction. This video clip explains three types of coronavirus testing. The first part explains the PCR testing described above: