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At POC and in lab, 2 new checks on SARS-CoV-2 testing

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Valerie Neff Newitt

November 2020—The CAP released in September its proficiency testing program for SARS-CoV-2 antigen testing, with the first shipment to laboratories set for Nov. 30.

It also introduced recently a Quality Cross Check program that makes it possible for labs performing nucleic acid amplification testing for SARS-CoV-2 to monitor performance across multiple instruments, in compliance with the CMS directive prohibiting proficiency testing on multiple instruments.

“Even with its limitations, antigen testing could have a role in a well-designed testing strategy, but nucleic acid testing remains the standard of care for a definitive diagnosis,” says Daniel D. Rhoads, MD, member of the CAP Microbiology Committee, which helped to develop the proficiency test. With laboratory medicine having “pushed to the forefront of news” during the pandemic, “it’s critically important that we are performing well in all testing formats and meeting the need in this time of crisis,” says Dr. Rhoads, assistant professor and section head of microbiology at the Cleveland Clinic.

The latest proficiency test release follows the release last spring of PT for NAAT and for serologic tests that detect the presence of antibodies against SARS-CoV-2. Laboratories that enroll in the antigen PT will receive several samples with inactivated virus “containing all of the antigens that could ever be tested for in SARS-CoV-2,” says Dr. Rhoads, adding it will work with all antigen assays, including those yet to come.

Dr. Rhoads calls the antigen PT and the other CAP SARS-CoV-2 proficiency tests “an opportunity for labs to challenge the system—both the performance of the assay and the technologists doing the tests—and compare themselves to their peers.”

He offers a few cautions about antigen testing for SARS-CoV-2. The tests on the market (as of September) are designed to be performed with a dry swab. “It needs to be collected and tested oftentimes within an hour of that collection, depending on the manufacturer. Different manufacturers have different instructions that have been authorized by the FDA, so not all of them are only an hour. You need to know that before you order the test and put all your eggs in that basket. You want to make sure you know the limitations.”

If these tests are performed in the laboratory and not at the point of care, Dr. Rhoads says, there is often no good way to transport them. “When we test the PCR nasopharyngeal swab and put it in transport media, we screw the lid on tightly and then everything is contained.” The dry swabs can be slipped back into the sleeves they came in and placed in a bag, “but it’s a little less tidy. It’s a concern,” he says.

Most antigen tests are designed for use at the point of care. “When that dry swab is tested, the specimen is used up. You cannot then reflexively test for flu or RSV or other respiratory viruses or even retest for SARS-CoV-2 off the same specimen. You would need a new specimen.”

Antigen tests are not used at the Cleveland Clinic, where the focus has been to increase capacity for NAAT. Fast turnaround time is the allure of the antigen tests, but knowing what to do with the result can be a problem, Dr. Rhoads says.

Dr. Rhoads

“If a provider orders a test but cannot be confident of the results, it often leads to retesting until there is greater confidence. So we are thinking hard about the problem we’re trying to solve and then trying to build the best solution to meet the need.” With more data and a better understanding of how the antigen tests perform, he says, the decision about whether to use them may change. “But as it is right now, I do not see a compelling use case.”

When it comes to testing for SARS-CoV-2, “everything has pros and cons,” Dr. Rhoads says, noting that the FDA authorized antigen testing for testing symptomatic people, making it inappropriate for screening, despite the market for such. “Different assays have different criteria as to how long after symptom onset you’re able to test someone and report results as presumptively negative,” he says. Insufficient data has been collected to date in independent studies about antigen testing for SARS-CoV-2. “But with experience with antigen testing for other respiratory viruses, we know it is generally less sensitive than a nucleic acid amplification test.” Thus, some false-negatives are expected.

“But we don’t expect to see false-positives, and yet there have been anecdotal reports of false-positives.”

“We need clarity,” through clinical studies, Dr. Rhoads says, on “whether there is a problem with human error, instrumentation, or the assay itself. We must consider how confident we are in results being accurate.”

For now, he says, antigen tests are believed to be best used in sympto­matic people shortly after symptom onset when the antigen load would be its highest and it would be easiest to detect.

“The more tools we have the better,” Dr. Rhoads says, “and certainly there are advantages to an antigen test.” But it is matching the best tool or solution with each need—screening, diagnosis, surveillance—that is the challenge.

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