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Missouri health department: Current positivity rate 'cannot be compared' to old method after reporting change

The change accounts for one person being tested more than once, which would have resulted in a lower rate
Credit: KSDK

JEFFERSON CITY, Mo. — The Missouri Department of Health and Senior Services said a change in the way it reports COVID-19 testing data changed the way the state calculates the positivity rate, and that the new calculation "cannot be compared" to the rates reported as recently as Friday.

According to a news release from the health department, the positivity rate was previously calculated as the number of positive COVID-19 cases divided by the total number of tests completed. The new calculation takes the number of positive cases and divides it by the total number of people tested. The change accounts for one person being tested more than once.

The news release said the change was made because the previous calculation increased the denominator but not the numerator, which would lower the rate.

The current positivity rate using the new data is 7.74%. According to data collected by the Associated Press COVID Tracking Project, the positivity rate using the numbers reported by the state on Friday would have been 6.68%.

Positivity rates calculated by the new calculation are highlighted on the graph below.

Credit: KSDK

“As we continue to learn more about this virus and new tests emerge, we will continue providing better data with greater clarity and transparency to help Missourians make the best decisions for their health care possible,” DHSS Director Dr. Randall Williams said in a news release.

A low positivity rate can be a sign that the state has sufficient testing capacity and is able to test more than just the sickest in the population. The World Health Organization says the goal should be 5% or lower. The CDC and White House both say states should see positivity rates trending down in order to safely loosen restrictions meant to limit the spread of COVID-19.

The news release from the Missouri Department of Health and Senior Services said the department was following reporting guidelines as requested from the Centers for Disease Control and Prevention but made the change to provide more accuracy and clarity for Missourians. 

RELATED: CDC, states' reporting of virus test data causes confusion

In Missouri, the change was made the same day the department started reporting data on serology testing — also known as antibody testing. The health department said the number of tests reported by the state decreased Saturday because the department would "no longer including people who received only serology tests in this graph."

A health department spokeswoman said people who test positive with a serology test have never been included in the state's count of positive cases.

On Monday, the department reported 12,167 COVID-19 cases and 685 deaths, an increase of 179 cases and four deaths. The state also reported an increase of 5,512 new PCR tests, bringing the state's total to 157,131.

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