It’s a small change that is making a difference in the way the Indiana State Department of Health is reporting COVID-19 data to the public. And it comes down to two different tests: PCR and antigen, which work differently in diagnosing the disease.
But will the results be as meaningful?
Starting Monday, the state began including positive results from both PCR and antigen tests. It previously had reported only the results of the polymerase chain reaction, or PCR, tests. That single move reduced the state’s positivity rate—or the percentage of people who test positive for the virus of all those who have been tested.
“When we first started, the only approved test to diagnose the active viral infection was the PCR test, which identifies the genetic material of the virus,” Dr. Kristina Box, Indiana State Health Commissioner, said last Wednesday during Gov. Eric Holcomb’s weekly press briefing. “Since then, an additional test to diagnose active infection of COVID-19, called an antigen test, has become widely used. The antigen tests check for a protein on the outside of the virus.”
Both of the tests can be obtained through nasal swabs, and both can be either sent to laboratories for analysis or can be run through a quick analysis at the testing site while a person waits.
Historically, Indiana has included only PCR tests on the state health department dashboard. The U.S. Centers for Disease Control and Prevention defined a COVID-19 case as one that was documented by the PCR test. But that recently changed.
“Now, the CDC recognizes the need to document a case of COVID-19 also using the newer antigen test,” Box said.
How accurate is the antigen test?
The first antigen tests gained Food and Drug Administration approval on May 8. The company, Quidel Corp., said it would use the first 40,000 tests to get a better idea about the accuracy of its test, according to a report on National Public Radio. Researchers said they did not expect it to be as accurate as the PCR diagnostic test, but it is possible the antigen tests could be used to screen patients for infection, the report said.
The Mayo Clinic says that a positive antigen test result is considered very accurate. “But there’s an increased chance of false-negative results—meaning it’s possible to be infected with the virus but have a negative result,” the clinic reported. “Depending on the situation, the doctor may recommend a PCR test to confirm a negative antigen test result.”
And Harvard Medical School says that the rate of false negatives from antigen tests can be as high as 50%, which is why they are not favored by the FDA as a single test for active infection. “However, because antigen testing is quicker, less expensive, and requires less complex technology to perform than molecular testing, some experts recommend repeated antigen testing as a reasonable strategy.”
The CDC says the clinical performance of rapid antigen diagnostic tests “largely depends on the circumstances in which they are used.”
“Rapid antigen tests are particularly helpful if the person is tested in the early stages of infection with SARS-CoV-2 when viral load is generally highest,” the agency says on its website. “They also may be informative in diagnostic testing situations in which the person has a known exposure to a confirmed case of COVID-19.”
On Monday, the Indiana State Department of Health included 16,215 antigen tests and 975 positive cases from July 28 through Aug. 22 to its cumulative reports.
The inclusion of antigen tests is already beginning to alter the overall and seven-day testing-positivity rates that the department reports on a daily basis. On Monday, the state reported that the positivity rate for unique tested individuals over seven days was 7.2%, down from 7.5% on Sunday. The overall positivity rate for unique tested individuals dropped from 8.8% to 8.7%.
“Moving forward, antigen cases and testing from the most recent 24-hour period will be included in the daily reports,” the state said. “Tests and cases from prior to July 28 will be added to the cumulative reports once they are verified.”