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Case Positivity vs Test Positivity | Covid Terminology

Much reporting on covid merely states "positivity rate" but does not clarify the methodology for the percentage they find.

Jim Spaniol reports COVID data for Macon County on facebook and brought this to my attention. I am Reed Sutman & wrote this page on March 16, 2022.


On Monday, Gary & 9 other people are tested for COVID. Only Gary tests positive. Case positivity for Monday is 10% AND test positivity is 10%.

On Tuesday, Gary & the other 9 people are tested again. Gary STILL tests positive. Everyone else tests negative again. Case Positivity for Tuesday is 0%, because there are no new cases. Test positivity is 10%, because 10% of tests are positive

The difference

Case positivity measures the number of new cases, among the people tested. Test positivity tests the number of positive tests, regardless of prior positive/negative tests.

In Jim's Words

Case positivity - 10 people tested. One of the people tested has a new case of COVID so case positivity is 10%. (Not all people who test positive are listed as a new case. This happens when a person who has already been tested and counted as a case, gets tested again and is still positive. It's a positive test but not a new case.)
Test positivity - 10 people are tested and 1 person tests positive. Test positivity is 10% regardless of the state of any of the people prior to that test.
Better example -
Ten people are tested today. Of those ten, 9 people are not current cases because they haven't tested positive for two months. Some may have previously had COVID and were counted as a case at that time but they haven't tested positive for two months so they are not a "case" now. One of the 10 people tested positive for COVID last week. Last week that increased the case positivity because the person was not previously a case but then was a case. Today that person is still a case because the time period hasn't been long enough for her to be considered "not a case".
Test results are that of the 9 people who were "not a case" one tests positive and 8 test negative. Case count is increased by 1.
The person who was a case tests positive again. This makes 2 positive TESTS out of the 10 tests done so Test positivity is 20%. The person was already a case so case positivity does not increase. There is a total of 1 new CASE out of the 10 tested so Case positivity is 10%.
The distinction is important because it depends on what "problem" you want the metric to help people understand. If you want the medical community to understand the current number of people in Macon County that have COVID then you want to tell them about test positivity. After all in our sample of 10 people 2 of them have COVID right now. So the inference is that of the entire population of Macon County there are more people who have COVID now than when test positivity was at 15%.
On the other hand if you want people to understand the likelihood that they might get a case of COVID, you would want to report a metric related to how many new cases we are seeing. You would report case positivity for that purpose.
If you wanted to be thorough, you would report both case positivity and test positivity. However if you were a news media outlet you might think that this would be confusing to your audience. You also might want to report sensational news. So you might report whichever number is higher and call it just "positivity". If you wanted to be thorough and you were also a health agency that wanted to provide the best possible information to people you might report both case positivity and test positivity AND explain to the public what these terms mean and how to correctly interpret and use the information to make good health decisions.