Your Coronavirus Test Is Positive. Maybe It Shouldn’t Be.
Some of the nation’s leading public health experts are raising a new concern in the endless 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, 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 that it would purchase 150 million rapid tests.
But similar PCR tests for other viruses
do 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 — that’s
all,” Dr. Mina said. “We’re using that for clinical diagnostics, for
public health, for policy decision-making.”
But yes-no isn’t good enough, he added. It’s the amount of virus that
should dictate the infected patient’s next steps. “It’s really
irresponsible, I think, to forgo the recognition that this is a
quantitative issue,” Dr. Mina said.
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
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.
One
solution would be to adjust the cycle threshold used now to decide that
a patient is infected. 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.
Tests
with thresholds so high may detect not just live virus but also genetic
fragments, leftovers from infection that pose no particular risk — akin
to finding a hair in a room long after a person has left, Dr. Mina
said.
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.
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Credit...Erin Schaff/The New York Times
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