“Our statement will be very clear that both probable and confirmed cases should be reported to the CDC and counted in a universal way,” Engel told MedPage Today.
The council’s 10 board members voted on the guidance earlier this week.
“A (positive) antigen test will be counted as a probable, and a PCR test will be counted as confirmed,” he said. Engel added that states can decide whether to lump them together or separate them out as “confirmed” or “probable,” but the council’s advice will be to include them one way or another.
“States that don’t choose to do that I think are going to put themselves in a lot of risk for not being transparent. And I think it will cause more confusion,” he said.
Big states like California, he said, “I expect will begin to pivot and report these probable cases.” Engel said that the CDC has agreed to the new reporting protocols.
In a follow-up call, Engel said that if a state decides not to report probable antigen tests in its case counts, it should explain its reasons why. It may be that they don’t trust the tests’ accuracy. But he expects most states, like California and Texas which do not include these cases in daily counts now, “will pivot and report these probable cases.”
Proportion of Rapid Tests Growing
Two rapid antigen tests — promising on-the-spot results in minutes, rather than hours or (more frequently) days — have received emergency use authorization (EUA) from the FDA for people with symptoms: the
Sofia Sars Antigen FIA by Quidel and the
BD Veritor System by Becton Dickinson.
Both are now rapidly being adopted around the country for fast, point-of-care use. Quidel, for instance, promotes its rapid test for groups such as healthcare workers and first responders.
Some public health officials have urged that they be used more widely, in response to problems with traditional PCR tests that now may take more than a week for results to come back. The painfully slow turnaround is hindering individual patient management and contact tracing as well as preventing public health officials from getting a clear picture of their rapidly evolving situations.
Engel declined to say how much he thinks daily case counts from states will grow when the guidance is adopted, because it depends on how many machines are available. “It’s growing all the time,” he said, with shipments often handled by the Department of Health and Human Services.
The council’s
initial guidance on laboratory criteria for reporting cases of SARS-CoV-2, issued in early April, included a recommendation to count as cases positive results from clinical specimens that use viral molecular amplification as well as those that detected specific antigen, a protein from the virus.
But in routine calls to the states, Engel said, the council made it clear that April guidance applied only to tests performed on autopsy tissue “in the early days of March and April when people were dying at home or dying in emergency departments before they could get care.”
Tests weren’t widely available, particularly in New York City, he said, where death counts were rapidly rising.
While the tests have EUAs, they are far from perfect compared with PCR tests.
“It depends on the setting, but (antigen tests) can be as high as a 50% false negative,” said Engel, meaning that it will miss a lot of infected patients.
However, it has a much better reliability when an antigen test comes back positive.
“At the bedside, if you’re a physician and you suspect a person to have COVID-19, if their antigen test is negative it’s recommended, even in the package insert, to do a PCR test,” Engel said.
The antigen tests provide a less invasive way of obtaining a specimen, with a simple nasal swab instead of the deep nasopharyngeal probe. Many infectious disease experts welcome them as a way to rapidly test large populations at risk, such as students, sports teams or even members of a faith congregation, before they enter a building.
Geoffrey Baird, MD, PhD, interim chair of laboratory medicine at the University of Washington in Seattle, a hospital that does not do antigen testing, is realistic about the utility of these rapid assays.
“Their positive predictive value is quite high, especially in an area where there’s a lot of prevalence in the community, there’s a higher chance that a positive test is a real positive,” Baird said.
But Baird wondered how well it will work “when you bring this into the real world. Because one of these problems with these (antigen) tests is false negatives. How good of an idea do you think it is to tell a college student that they’re negative?” he asked. “Are you really sure of that answer, or are you only 80% sure?”
Six Months In, So Much Confusion
It also depends on what part of the body the virus has infected. It may be that in some people, the virus and the proteins that the antigen test measures, are “chock full in the lungs, but not in the nose.” That’s going to be the case when you test that person repeatedly, and “you’re going to miss it.”
The other “real world” problem, Baird said, “is that the Holy Grail [for] a lab test is that it’s cheap, fast and accurate. The problem is for any given test you can only have just two of those. You can pick which two, but it makes the other unachievable.”
Ravina Kullar, PharmD, of the David Geffen School of Medicine at UCLA, acknowledged that so many people, including health professionals, are confused about the topic.
“I’m on the front lines, seeing patients in nursing home settings for infection control,” and some of those facilities had outbreaks. It’s taking a week, and sometimes longer, to get a result from a PCR test for COVID-19 in their residents, during which everyone, including the staff, is worried, she said. A rapid test would be so much more helpful.
“It’s so unfortunate that we’re six months into this epidemic, and there’s so much confusion, we don’t know what to do,” and that confidence in antigen test accuracy is just not there yet, she said.
But when antigen tests are ultimately embraced around the country, Kullar said, “I think workplaces, in sports, maybe perioperative care as well, those settings will be able to use it.”
Engel acknowledged that many states are under a lot of pressure right now and that “they have no idea what their political leadership is going to do.”
Asked if he fears that some states may choose not to report antigen positive test results for COVID-19 in their case counts because it won’t make their state look good, Engel replied: “I suppose that’s possible but we’ll deal with that one state at a time.”
