Maryland Health Department Measures Key COVID-19 Metric Differently Than Johns Hopkins "Our team has had multiple conversations with [Johns Hopkins] ... they've actually stated to us that our methodology is not wrong, it is different," said deputy health secretary Jinlene Chan.
From NPR station

WAMU 88.5

NPR logo Maryland Health Department Measures Key COVID-19 Metric Differently Than Johns Hopkins

Maryland Health Department Measures Key COVID-19 Metric Differently Than Johns Hopkins

Virginia National Guard Soldiers and Airmen assist with a drive-thru COVID-19 testing site run by the Virginia Department of Health Eastern Shore Health District on May 8, 2020. Virginia Guard Public Affairs/Flickr hide caption

toggle caption
Virginia Guard Public Affairs/Flickr

Maryland lawmakers pressed the state's health department leadership over data inconsistencies in a meeting on Wednesday. Many of their questions had to do with the test positivity rate, a key metric recommended by the World Health Organization for making reopening decisions.

As of Wednesday, Maryland's coronavirus website says the percentage of tests coming back positive is 3.6%. On Friday, Maryland Governor Larry Hogan put out a press release celebrating the state's declining COVID-19 test positivity rate, remarking that it had dropped below 4% for the first time since the state began measuring it.

But, as several lawmakers pointed out in Wednesday's meeting, the Johns Hopkins Coronavirus Resource Center says Maryland's percent positive rate for tests is currently 5.2%.

Public health experts have repeatedly emphasized the 5% threshold. The WHO has told governments they should have a test positivity rate lower than 5% for two weeks before reopening. Additionally, the federal government instituted a requirement last month for all nursing homes "in states with a 5% positivity rate or greater" to test staff on a weekly basis.

Article continues below

During the Wednesday briefing, Maryland Senate President Bill Ferguson said it was "crucial" for decision makers to get more clarity on the discrepancy.

At one point, Del. Shane Pendergrass of Howard County expressed her confusion with both the difference in the test positivity rate, as well as the fact that the number of confirmed cases and negative test results on Maryland's dashboards do not add up to the total number of tests administered. (As of Wednesday, there was a difference of more than 400,000 tests.)

"Can you explain why this is so not straightforward? It seems cryptic, or — there's something missing," asked Pendergrass.

"What we have tried to do, and we've been consistent at the health department since the very beginning, is we tried to pick numbers that we thought could be as consistent as we can make them," responded Dr. Jinlene Chan, the state's deputy health secretary.

Chan said the positive and negative results might not add up to the total number of tests because of "data reconciliation issues," or because a certain number of test results might be indeterminate.

As for the positivity rate, Chan said the difference could be explained by the fact that the Maryland health department calculates test positivity differently than Johns Hopkins.

"Our team has had multiple conversations with the Hopkins Coronavirus Resource Team ... and they've actually stated to us that our methodology is not wrong, it is different, and they acknowledge the differences in our methodologies," said Chan.

According Maryland's state website, the positivity rate they publish "is a seven day rolling average of positive results as a percentage of all tests." Chan said in Wednesday's meeting that the state is dividing the total number of positive tests by the total number of tests reported in a day to calculate the metric.

Hopkins also publishes a seven-day rolling average. But it calculates positivity rate by dividing the number of people who test positive by the number of people who get tested, a method they say rules out "duplicative tests."

Health departments in Maryland — as well as across the region and the country — are prominently displaying data on their coronavirus websites. Many jurisdictions in the area post dashboards with key metrics they say they are using to make decisions about reopening the economy. In some cases, jurisdictions like D.C. have moved forward with reopening plans despite failing to meet the benchmarks they set for themselves.

The briefing with Maryland lawmakers on Wednesday underscored that despite the central role of data in the pandemic response, there is persistent confusion around methodology and data practices.

While measuring the COVID-19 pandemic is crucial, it has also been difficult since the beginning, as Ed Yong reported in The Atlantic in April.

"I think people underestimate how difficult it is to measure things," Natalie Dean, a statistician at the University of Florida, told The Atlantic. "For us who work in public health, measuring things is, like, 80 percent of the problem."

Questions or comments about the story?

WAMU 88.5 values your feedback.

From NPR station

WAMU 88.5