Covid-19 antigen tests were not counted among the cases in some states, CNN analysis shows

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Five states – California, Colorado, Maryland, Missouri and Nevada – publicly report only cases confirmed with PCR testing in their online statements, according to CNN’s analysis of health department dashboards in all 50 states.

Some states only report polymerase chain reaction test results in their Covid-19 case numbers and exclude antigen test results, as antigen testing is generally considered less accurate. But as antigen testing grows in the United States, this exclusion could lead to potential miscalculations.

In California, for example, CNN was told that antigen Covid-19 test results should be reported to local health departments and to the California Department of Public Health. But publicly reported Covid-19 test data only reflects the results of diagnostic tests that detect the genetic material of the coronavirus, such as PCR – polymerase chain reaction tests.

CNN requested antigen testing data from the state department of public health, but was informed in an email that “antigen test results are not currently publicly reported statewide.”

In the four remaining states, the data they each shared with CNN represents approximately 10,000 positive Covid-19 antigen test results reported over just a small sample of days in January.

If included in the reported cases, according to CNN’s analysis, those positive antigen test results would increase Covid-19 cases reported by those states by 5% to 50% over that time.

In Maryland, 3,522 positive antigen tests were reported to the Maryland Department of Health between Jan. 23 and Jan. 29, the department told CNN in an email. That works out to about 503 positive antigen tests per day that are not included in the state’s daily dashboard of cases.
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In Nevada, a total of 7,764 antigen tests were reported to the Nevada Department of Health and Human Services between Jan. 21 and 27, according to a document from the department’s Office of Analytics. Of those tests, 623 were positive. In Nevada, state counts are based on “ molecular tests only, ” with the exception of the Nevada Department of Corrections, which uses the BinaxNOW antigen test on people who are currently incarcerated.

In Colorado, there were a total of 10,203 antigen tests per day between Jan. 25 and 31, 527 of which were positive, according to the Colorado State Emergency Operations Center. The department told CNN in an email that the seven-day mean positivity for antigen tests over the past week was 5.17%.

In Missouri, a total of 50,113 antigen tests were reported to the Department of Health during the week of Jan. 10, according to data obtained by CNN. Of those tests, 5,285 were positive.

‘We will see an underestimate’

Public health officials have noted that while antigen testing is usually faster and cheaper than typical PCR testing, antigen testing is more likely to miss an active Covid-19 infection when compared to PCR testing. Because of this difference in accuracy, some states choose to only report Covid-19 cases confirmed with PCR testing.
In June, at least 28 states followed the CDC’s recommendations to report antigen test results as “probable” cases in the number of Covid-19 cases. While the number is now much smaller, some experts are concerned that not reporting “likely” publicly in the future could bias the true spread of disease to residents of a state.
“Antigen-based tests aren’t necessarily the majority of all tests done everywhere, so we’re unlikely to get a dramatically misjudged estimate from now on – also because most states are reporting the risk going forward,” Dr. David Dowdy, an infectious disease epidemiologist at the Johns Hopkins Bloomberg School of Public Health, told CNN last week.

“I think more and more of these antigen-based tests are being used,” he said. “So in the future there is certainly a risk that if these are not reported consistently, we will see an underestimate of the number of Covid cases in the country, and especially in the states that do not report them.”

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Faster antigen testing is expected in the United States, as last week the Biden government announced that the Department of Defense and the Department of Health and Human Services are working with Australian company Ellume to deliver more of its full-home Covid-19 tests to the United States. The test is an over-the-counter, rapid self-test that detects SARS-CoV-2 antigen. SARS-CoV-2 is the name of the virus that causes Covid-19.

While antigen tests work by detecting a protein that is part of the new coronavirus, PCR tests work by detecting genetic material from the virus called RNA.

For antigen tests, “proteins live in the body for a shorter time and thus more false negatives may come out of these tests,” wrote Dr. Elizabeth McNally, director of the Center for Genetic Medicine at Northwestern University Feinberg School of Medicine, in an e-mail. mail to CNN.

“Antigen tests are routinely used to detect other viruses, and we know they also contain false positives and false negatives,” she added.

Because PCR tests are usually very accurate, they do not need to be retested.

However, for antigen tests, “positive results are usually very accurate, but false positives can occur, especially in areas where very few people have the virus,” the US Food and Drug Administration said. Negative results may need to be confirmed with a PCR test.
Since antigen tests are more likely to report false negative results than false positive results, “the concern is more that there are people who have Covid that are missed by these antigen-based tests, but not that they are creating false positives,” Dowdy said. He added that it is estimated that antigen tests will be positive in about 80% to 90% of people with Covid-19.
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“So if what you’re mainly tracking is the percentage of your tests that are positive, then there is a risk that the percentage of positive will be underestimated by including the antigen-based tests,” said Dowdy. “But if you look at the total number – that’s generally what more people are looking at – and by excluding people who test positive, you actually have people who confirmed Covid-19 that aren’t being counted.”

In some states, antigen test results may be included in the total number of cases as “probable cases,” but not included in the percentage of test positivity, which tracks the percentage of all coronavirus tests performed in a state that are actually positive .

In Hawaii, for example, “antigen tests are counted as probable cases rather than confirmed cases,” Brooks Baehr, a spokesman for the Hawaii State Department of Health, told CNN in an email. But “to test percent positivity, antigen tests are not included in our listings because we lack accurate counts of the total number of antigen tests performed in our state.”

‘The more we test, the better’

“It seems odd” not to include the antigen test results in the statewide counts of Covid-19 cases, McNally of Northwestern University said in her email.

In the conclusions, “combining PCR and antigen could improve the sensitivity and specificity of assays, but overall the assays we have work quite well, so I don’t think this is really necessary,” McNally said. “We know there are far more cases of Covid-19 than reported.”

The main take-away, Dowdy said, is that Covid-19 testing should continue and remain an important tool in responding to the pandemic.

“The more we test, the better,” said Dowdy of Johns Hopkins University.

“It’s better for people to know whether they have this disease or not,” he said. “It is also important for public health authorities to know how many people have Covid-19 in a particular place – and so we should try to ramp up testing, both from PCR and the antigen-based tests, and make sure that we report them as completely as possible. “

CNN’s Virginia Langmaid and Deidre McPhillips contributed to this report.

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