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Antigen testing: All about the rapid test NH now uses in its COVID counts

The Keene Sentinel
Published: 10/23/2020 1:28:33 PM
Modified: 10/23/2020 1:28:23 PM

At the beginning of October, when New Hampshire began counting antigen COVID-19 tests as positive, health experts nationwide were scrutinizing the rapid tests’ accuracy.

However, preliminary data on antigen tests suggest that if done with the right population at the right time, the results — while still less reliable than a PCR (polymerase chain reaction) test — are accurate enough to be useful, according to Dr. Aalok Khole, an infectious disease physician at Cheshire Medical Center in Keene.

The N.H. Department of Health and Human Services’ decision to include antigen test results in its COVID-19 data reflects the tests’ increased availability, as well as officials’ aim to be transparent amid the pandemic, Jake Leon, a spokesman for the state agency, said Tuesday.

“Given the increasing availability of the test ... not capturing that data, not reporting out that data, I don’t think would have provided the most accurate picture of how the virus is spreading throughout the state,” he said.

New Hampshire has seen a spike in its number of known COVID-19 cases in recent weeks. As of Wednesday, there were 798 current cases, compared to 258 current cases on Aug. 21.

Some of that is likely due to an increase in PCR testing as well as antigen tests. But not all of it, according to Leon.

“We are still seeing a lot of testing, and the more testing you do, the more cases you will identify, but it’s clear there is more community transmission, not just more testing,” he said.

Four types of antigen tests have been approved by the Food and Drug Administration to date, Leon said.

New Hampshire learned at the end of September that it would receive shipments of antigen tests, he added. The federal government is expected to provide 400,000 — or 25,000 per week — Abbott BinaxNOW Rapid Antigen tests to the state by the end of the year, Leon said.

Also known as rapid tests, antigen tests use a mucus sample from the nose or throat to reveal whether a person is currently infected with the virus that causes COVID-19 by detecting certain proteins of the virus.

Antigen tests can yield results in as little as 15 minutes, according to Leon.

The tests are being given to community testing sites, he said. Locally, they are available at Cheshire Medical Center and Convenient MD, both in Keene, and Valley Regional Hospital in Claremont.

Antigen tests are recommended only for people showing symptoms of the viral disease — such as fever, cough, shortness of breath and loss of taste or smell — and should be done within five to seven days of symptoms arising.

The other option is a PCR test, performed most commonly with a six-inch nose swab, which can be taken at testing sites across the state. The test checks samples from someone’s respiratory system, and detects genetic material of the virus.

Result times for the PCR test vary, with some available in less than an hour and other tests taking one to two days to verify in a laboratory.

According to the FDA, both tests are considered “highly accurate.” However, antigen tests are more likely to miss an infection, so negative tests may still need to be confirmed with a PCR test.

Leon said the benefit of the antigen test is in its speedy results, especially if people need to get back to work or school.

“We can identify the active infection and then do contact investigations with the individual more rapidly, and if it’s negative they can return to their daily lives much quicker,” he said.

Dr. Khole, of Cheshire Medical, said he agrees with the state’s decision to include antigen positives in its data, as it “will give a true sense of the burden of illness in the community.”

However, he thinks it’s effective only if the state tracks the negative results too to properly calculate the testing percent positivity rate — the percentage of all New Hampshire COVID-19 tests performed that are positive.

This metric is critical because it shows how widespread infection is in the area where the testing is occurring, and whether levels of testing are keeping up with levels of disease transmission.

Leon said because the antigen tests are point-of-care tests — tests that are analyzed on site, such as at a hospital — the state doesn’t have an automated system in place as it does with other infectious diseases or the PCR tests, which are sent to a laboratory.

The state is currently working to address this, he added, so that both negative and positive antigen results are submitted. Because of this, Leon said New Hampshire’s positivity rate includes only PCR test results until a better system is in place.

The state’s latest daily PCR test positivity rate was listed Thursday at 0.8 percent.

“We are in the process of building that out and making it as easy as possible,” Leon said, “so that we can know the positivity rate for antigen testing.”


These articles are being shared by partners in The Granite State News Collaborative. For more information visit collaborativenh.org.


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