A key metric is cropping up more and more at the coronavirus task force briefings.

White House response coordinator Deborah Birx on Wednesday and Thursday highlighted what she called the positive rate of coronavirus tests, a figure that helps measure the severity of the outbreak in different parts of the country.

Why is this measure important? While much attention has been placed on the number of tests being conducted, the proportion of tests that come back positive can offer both a broader and more detailed view of the pandemic.

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“Epidemiology is a bit like baseball,” two public health experts, Farzad Mostashari and Ezekiel Emanuel, recently wrote in the news publication Stat. “Knowing that a ball player has gotten 134 hits isn’t that informative. What is informative is knowing that those 134 hits were made during 335 at-bats, which translates into a batting average of .400.”

Knowing how that percentage of positive tests is changing can help determine the course of the outbreak, they wrote.

Birx used the measure Wednesday, along with the number of new cases per day, to contrast the severity of the outbreak in two of the biggest U.S. cities.

New York had a positivity rate of more than 40 percent, with 11,000 new cases per day. In contrast, Los Angeles, which along with much of the West Coast is seen as a relative success story, had a positivity rate of just 9 percent and 800 new cases.

“And so this really gives us some idea of what it takes,” Birx said, pointing to Los Angeles. “They have been continuously mitigating.”

Areas at risk of becoming the next epicenter had positivity rates between those of New York and Los Angeles, effectively serving as a warning sign. New Orleans had a positivity rate of 28 percent, followed by Detroit at 26 percent and Chicago at 18 percent.

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The metric can also be read as an indicator that the country is not doing enough testing.

Nationally, the positivity rate of coronavirus tests is about 20 percent. Ashish Jha, director of the Harvard Global Health Institute, said that is “absolutely” a sign of insufficient testing.

He also noted that the positivity rate has increased in the past two weeks, an indication that the U.S. is not ramping up its total number of tests fast enough to keep pace with the spreading virus.

“A very high percent positive rate means you're probably missing a lot of people who have the disease,” he said, suggesting New York may have even more unconfirmed cases.

But in places such as Los Angeles, where the rate is much lower, that is a sign “there probably aren’t a lot of cases that I’m not seeing.”

Los Angeles County on Friday extended its stay-at-home order to May 15. It was previously scheduled to end on April 19.

The positivity rate certainly has its limits as a metric, though, mainly because it depends in large part on who is getting tested.

The bulk of testing is not being carried out in a systematic or randomized way that would best answer questions such as how far the virus has spread in a community.

“It can be so strongly influenced by just exactly what are the criteria for testing,” said Jeffrey Martin, a professor of epidemiology and biostatistics at the University of California San Francisco.

He said better metrics for the severity of an outbreak are deaths and intensive care unit admissions.

To help improve the reliability of the positivity rate, Mostashari, a co-author of the Stat article, called for comparing test results from a “consistent setting,” such as the emergency room, as a more standardized metric than the current “haphazard” testing analysis.

Mostashari, CEO of the health care company Aledade and a former New York health official, called for “testing consistently in those places apples to apples and then [comparing the] positivity rate.”

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He said the U.S. Department of Health and Human Services should put forward clearer requirements for labs to report details of their results to aid in that effort.

A clear picture of how many people in a community, even those who did not have symptoms, have had the virus likely will not come until a different kind of testing takes hold: blood tests known as serology tests. Those can determine if people have ever had the coronavirus, to measure the prevalence of the virus in a community.

“What we really are all waiting for is seroprevalence,” said Donald Thea, a professor of global health at Boston University.

Birx made a similar point at the White House on Friday by pointing to data that could come from a finger-prick blood test being developed.

“That’s a question we still have,” she said. “Is this the tip of the iceberg, or is this half the iceberg or three-quarters of the iceberg, what we have seen to date?”