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States undercount positive rapid tests, masking spread of disease

As rapid coronavirus tests become more widely available, delivering results within minutes for patients in doctors’ offices, nursing homes, schools and even the White House, officials warn against a significant undercoverage, blurring the spread of the virus nationally and in communities where these tests are more frequently used.

Public health officials say antigen tests, which are faster than polymerase chain reaction (PCR) tests but less able to detect low levels of the virus, are an important tool in limiting the spread of the coronavirus. But they warn that with inconsistent public reports, the undercount of cases may worsen as more ‘point-of-care’ antigen testing, as well as DIY and home testing kits, arrive. on the market.

“We want to be sure that we don’t now say ‘there is no disease’ when there is a lot of disease. All that has happened is the science with which we identify it has evolved, ”said Janet Hamilton, executive director of the State Council and Territorial Epidemiologists, the group that helps the Centers for Disease. Control and Prevention to define cases of coronavirus.

Public health experts say more rapid coronavirus tests will lead to a greater undercount of coronavirus cases, even in states that attempt to count them publicly. What’s going on?

Some states do not count cases based on antigen testing

Despite advice from the CDC to report cases based on PCR and antigen testing, Washington, DC and seven states do not publicly share the number of cases for those with positive antigen tests, including California, the New Jersey and Texas.

Six other states keep these counts separate from their total number of cases, and most of them report them less frequently.


Public reporting of antigen positive cases

Note: Data as of October 29. Texas and Missouri publish data on antigen testing, but cases do not. Local health agencies can report cases of antigens in states where the health department does not.·Source: national health agencies

The differences between states are due in part to each state’s comfort level with rapid tests, which are not “confirmatory” like PCR tests because they can miss low levels of the virus. Yet most states treat antigen-positive or “probable” cases the same as “confirmed” cases, following interviews and contact tracing. And a growing number of states, including New Mexico, Oregon and Utah, include people with positive antigen tests in their total number of confirmed cases.

People who test positive in rapid tests represent a small but growing share of all publicly reported cases. For example, in Florida, positive cases of antigen tests account for about 4% of all cases reported since March, the majority of which are based on PCR.But on a daily basis, antigen tests contribute a more visible share, accounting for as much as a third of the cases reported in recent days.

These numbers are expected to increase as rapid tests purchased by the federal government reach schools and nursing homes across the country this fall.

And cases based on antigen tests may be even larger in countries where such rapid tests have been more abundant.

For example, at least 26 of Texas’ 254 counties report antigen-positive cases on local health department websites, and several reveal a growing reliance on rapid test results. The state health department is yet to report them.


Cumulative cases of PCR and antigens in some Texas counties

Source: local health agencies (case of antigens); state and local agencies (PCR)

In Brown County in west-central Texas, antigen testing has been widely available since the summer, so the county is releasing positive cases from rapid tests and PCR to provide a more complete view of the infection, said Lisa Dick, administrator of the Brownwood- Brown County Department of Health.

“If we were just publishing PCR tests, we would just give the community the idea that things are getting better,” Ms. Dick said. “And people make decisions based on this information, from leaders to individuals.”

In Taylor County, rural west Texas, people who test positive with rapid tests account for more than half of all cases, as tests are available at many emergency care centers across the country. county, said Dr Annie Drachenberg, medical director of Abilene-Taylor. County health district.

Patients also tend to prefer same-day results that antigen tests offer, Drachenberg said, noting that the turnaround time for PCR results peaked at two weeks in the summer before to return to a day or two this fall.

“Once you break that trust with a patient to get that response quickly, it‘s hard to regain that trust.”

Non-traditional testing centers can leave states in the dark

Whether states publicly report antigen-positive cases or simply track them internally, many public health officials say their counts are incomplete because they don’t know where rapid testing takes place in their jurisdictions.

And unlike laboratories that typically perform more complicated PCR testing, many “point-of-care” centers performing antigen testing, such as nursing homes, emergency care centers, and schools, do not realize that ‘they need to report lab data, or may rely on slower and less efficient methods such as phone calls and faxes.

“We don’t know for sure what we don’t know,” said Dr. Edward Lifshitz, medical director of the Communicable Disease Department at the New Jersey Department of Health, which does not publicly report antigen positive cases because the figure is incomplete.

Dr Lifshitz added that sharing data on antigen positive cases could make some areas of the state, where point-of-care testing centers report correctly, appear to be more disadvantaged than other areas. for which data is missing.

“It will sound like ‘boy, this part of the state is seeing an increase in cases’ when it really isn’t,” he said. “It’s just that they report what they are supposed to report.”

To avoid missing the results of the antigen tests, some health services had to do their own sensitization.

In Houston, where infections increased over the summer, health officials searched for rapid testing centers online, made phone calls and posted a “dear supplier” letter on their website. In New Jersey, officials collected the names of testing centers from manufacturers of antigen tests and the Centers for Medicare and Medicaid Services, which distributed rapid tests to nursing homes across the state.

But even then, the test centers can still be missed.

Since the end of September, Alabama has reported three spikes in older cases because health centers neglected to report their antigen test results to public health officials. An emergency care group was responsible for the largest increase, which was reported on October 23, after the addition of older cases nearly tripled the daily number.


New cases reported daily in Alabama




Includes antigen cases

from unspecified days

Includes antigen cases

from unspecified days

Includes antigen cases

from unspecified days


Note: the majority of probable cases of the condition are identified by antigen testing.·Source: Alabama Department of Public Health

Alabama health officials say that upon learning of the existence of new testing centers, they train them to share test data in an electronic format they can easily work with. Health officials across the country in states like California, Georgia, New Jersey, Oklahoma, and Texas are doing the same, but training can be labor-intensive and time-consuming, and many centers test do not have the technical support they need.

“It’s a pipeline that is not well established,” said Kirstin Short, chief of the epidemiology office at the Houston Department of Health.

Many other tests are in progress

Scientists following the development of coronavirus testing say rapid testing capacity – most based on antigen – could reach 200 million tests per month by early next year and help the country reach levels of recommended test.

As of September, the country reported more than 20 million completed PCR tests and about 5 million antigen tests, although the latter is a significant undercoverage, according to Mara G. Aspinall, a professor of practice in biomedical diagnostics at Arizona State University following Covid-19 test.

Antigen testing is also expected to expand beyond “point-of-care” testing to include portable kits that individuals can administer on their own. In fact, about two dozen companies are working on these personal rapid tests, according to Ms Aspinall.

Public health officials say these home tests, just like a pregnancy test, can be almost impossible to follow. Some experts have also expressed concerns that home testing might present its own pitfalls as large groups of people inexperienced in administering tests use the products and try to interpret their results.

“We could potentially get these tests over the counter,” said Dr. Lifshitz of the New Jersey Department of Health. “From a public health perspective, this is a good thing. From a surveillance point of view that becomes a nightmare.