Usual diagnostic tests may be too sensitive,

FDA says test with less sensitivity could miss those newly infected.

Some leading public health experts are raising a new concern in the debate over coronavirus testing in the United States: The standard tests are diagnosing huge numbers of people who may be carrying relatively insignificant amounts of the virus.

Most of these people are not likely to be contagious, and identifying them may contribute to bottlenecks that prevent those who are contagious from being found in time. But 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.

“The decision not to test asymptomatic people is just really back

ward,” said Dr. Michael Mina, an epidemiologist at the Harvard School of Public Health. “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 said Thursday that it would purchase 150 million rapid tests.

The mostwidely used diagnostic test for the new coronavirus, called a PCR test, provides a simple yes-no answer to the question of whether a patient is infected. But similar tests for other viruses offer some sense of how contagious an infected patient may be: The results may include a rough estimate of the amount of virus in the patient’s body.“We’ve been using one type of data for everything, and that is just plus or minus,” Mina said. “We’re using that for clinical diagnostics, for public health, for policy decision-making.” But yes-no isn’t good enough, he said: The amount of virus should dictate the infected patient’s next steps.

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 number of amplification cycles needed to find the virus, called the cycle threshold, is never included in the results sent to doctors and virus patients, although it could tell them how infectious the patients are. In three sets of data that include cycle thresholds, compiled by officials in Massachusetts, New York and Nevada, up to 90% of people testing positive carried barely any virus.

On Thursday, the U.S. recorded 45,604 new virus cases. If the contagiousness rates 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.

The Food and Drug Administration said in statement that it does not specify cycle threshold ranges used to determine who is positive — “commercial manufacturers and laboratories set their own.” The CDC said it is examining the use of cycle threshold measures “for

policy decisions.”

The FDA noted that people may have a low viral load when they are newly infected. A test with less sensitivity would miss these infections. But that problem is easily solved, Mina said: “Test them again, six hours later or 15 hours later or whatever,” he said. A rapid test would find these patients quickly because their viral loads would quickly rise.