Gothamist
In New York and beyond, the coronavirus pandemic has ushered in an
unusual new conversance with public health metrics, with government
officials now using testing and hospitalization data as a gauge for the
severity of the virus and important policy decisions.
No metric
has drawn more attention—or controversy—than the positivity rate. The
number, which measures the percent of tests that turn up positive and
serves as a proxy for infection rate, has been used to justify economic
reopenings as well as business and school shutdowns.
For example,
Governor Andrew Cuomo has said that he would order schools to close
should local positivity rate rises above 9 percent, an increasingly
likely possibility given the surge of infections.
In spite of the
high stakes, New York state officials have been calculating positivity
on a flawed basis, according to several experts. The criticism stems
from the state's decision to fold in a type of rapid test known as
antigen tests, which are less sensitive than polymerase chain reaction
(PCR) tests.
"That's not what the professional guidance is," said
Dr. Jay Varma, Mayor Bill de Blasio's top health adviser. "It's not what
the WHO does or the CDC or the Council of State and Territorial
Epidemiologists does."
The Council of State and Territorial
Epidemiologists, which advises the Center for Disease Control and
Prevention and other health agencies, has said that a positive antigen
test should be considered a probable case of COVID-19, not a confirmed
case. In turn, the CDC has defined its positivity rate as based on PCR tests only.
Despite
the limitation of antigen tests, their benefits are clear: the tests
can provide results within 15 minutes and can cost as little as $5,
whereas PCR tests cost around $100 at national labs. Antigen testing has
proliferated across the country in nontraditional testing centers, like schools and nursing homes, especially as the demand for testing increases.
In
New York City, the tests are now available at pharmacies, CityMDs and
doctor's offices. Drawn by their quick turnaround time, many people are
unaware of the caveats.
During a recent seven-day period, there
were nearly 88,000 antigen tests performed in one week, although city
health officials cautioned that not all antigen test results are being
reported to the city.
According to Cuomo, the state performs “hundreds of thousands" of antigen tests per week.
Dr.
Ian Lipkin, a renowned epidemiologist at Columbia University and
testing expert, agreed with Varma that antigen tests should be separated
from PCR tests. The latter are currently considered the gold standard
in terms of accuracy.
"You need to distinguish those clearly," he
said. "It's reasonable to say these are PCR, these are antigen, but I
don't think you should mix the two."