The most widely used diagnostic test for COVID-19, called a PCR test, provides a simple yes-no answer to the question of whether a patient is infected. But, a yes-no result isn’t really useful. It’s the amount of virus that should dictate the next steps for a patient infected with COVID-19.
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. This cycle threshold is never included in the results sent to doctors and coronavirus patients, although it could tell them how infectious the patients actually are.
Apoorva Mandavilli for The New York Times:
Researchers say the solution is not to test less, or to skip testing people without symptoms, as recently suggested by the Centers for Disease Control and Prevention. Instead, new data underscore the need for more widespread use of rapid tests, even if they are less sensitive. “The decision not to test asymptomatic people is just really backward,” said Dr. Michael Mina, an epidemiologist at the Harvard T.H. Chan School of Public Health, referring to the C.D.C. recommendation. “In fact, we should be ramping up testing of all different people,” he said, “but we have to do it through whole different mechanisms.”
In what may be a step in this direction, the Trump administration announced on Thursday [August 27] that it would purchase 150 million rapid tests…
“We’ve been using one type of data for everything, and that is just plus or minus — that’s all,” Dr. Mina said. “We’re using that for clinical diagnostics, for public health, for policy decision-making… It’s really irresponsible, I think, to forgo the recognition that this is a quantitative issue.”
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…
Most tests set the limit at 40, a few at 37. This means that you are positive for the coronavirus if the test process required up to 40 cycles, or 37, to detect the virus. 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. A more reasonable cutoff would be 30 to 35, she added. Dr. Mina said he would set the figure at 30, or even less.
Officials at the Wadsworth Center, New York’s state lab, have access to C.T. values from tests they have processed, and analyzed their numbers at The Times‘ request. In July, the lab identified 794 positive tests, based on a threshold of 40 cycles. With a cutoff of 35, about half of those tests would no longer qualify as positive. About 70 percent would no longer be judged positive if the cycles were limited to 30.
In Massachusetts, from 85 to 90 percent of people who tested positive in July with a cycle threshold of 40 would have been deemed negative if the threshold were 30 cycles, Dr. Mina said. “I would say that none of those people should be contact-traced, not one,” he said.
MacDailyNews Take: Making decisions with bad data has long had an acronym known by computer programmers: GIGO (garbage in, garbage out).
Making policy decisions — including closing schools, companies, retail stores, etc. — with bad COVID test data is a recipe for garbage decisions.
We’ve even got Apple closing retail stores based, at least in part, on COVID-19 case spikes. GIGO.
Importantly, this situation can be remedied. The report notes that the F.D.A. says people may have a low viral load when they are newly infected, so a test with less sensitivity would miss these infections. But that problem is easily solved, as Dr. Mina told The Times, “Test them again, six hours later or 15 hours later or whatever,” he said. A rapid test would find these patients quickly, even if it were less sensitive, because their viral loads would quickly rise.
What’s needed is a COVID test that’s fast, cheap, and pletiful enough to frequently test everyone who needs it — even if the test is less sensitive. “It might not catch every last one of the transmitting people, but it sure will catch the most transmissible people, including the superspreaders,” Dr. Mina told The Times. “That alone would drive epidemics practically to zero.”