IDSA Panel Updates Guidelines on COVID Molecular Diagnostic Tests

Marcia Frellick

January 06, 2021

Editor's note: Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

Saliva spit tests stack up well against the gold standard for molecular COVID-19 tests — the back-of-the-nose deep swab — without the discomfort and induced coughing or sneezing of the test-taker, updated guidelines indicate.

In a press briefing today, the Infectious Diseases Society of America (IDSA) explained the findings of an expert panel that reviewed the literature since the IDSA released its first guidelines in May.

The panel found that saliva tests were especially effective if the test included instructions to cough or clear the throat before spitting into the tube, said panel chair Kimberly E. Hanson, MD, MHS, of University of Utah Health in Salt Lake City.

Throat Swab Alone Less Effective

Using a throat swab alone was less effective and missed more cases than the other methods, she said.

The IDSA has updated its recommendation: A saliva test or swabs from either the middle or front of the nose front are preferred to a throat swab alone.

A combination of saliva and swabs from the front and middle of the nose and throat together "looked pretty much equivalent" to the gold-standard deep swab, the panel found.

She acknowledged, however, that multiple swabs exacerbate already challenging supply issues.

Saliva samples do come with challenges, Hanson noted. A laboratory must validate that its systems can handle the stickier material. And asking a patient to cough necessitates more personal protective equipment (PPE) for the healthcare professional.

Each center will have to tailor the specimen type it chooses, based on what resources it has available and the setting — whether in a hospital or a drive-through operation, for instance, she said.

Rapid Testing vs Standard

Panel member Angela M. Caliendo, MD, PhD, of Brown University in Providence, Rhode Island, said the panel preferred rapid polymerase chain reaction (PCR) tests and standard, laboratory-based PCR tests over a rapid isothermal test.

The panel defined rapid tests as those for which results are available within an hour after a test provider has the specimen in hand. They excluded home tests for this category.

The only rapid isothermal test that had enough data on which to issue a recommendation was the ID NOW test (Abbott Labs), she noted. 

Rapid PCR tests performed just as well as the standard laboratory-based tests, she said, with a high sensitivity of "97% on average and a very high specificity."

But the rapid isothermal test had an average sensitivity of only about 80% compared with the lab-based PCR test, Caliendo said, yielding a substantial number of false-negative results.

Testing centers will have to weigh the considerable advantages of having results in 15 minutes with a rapid isothermal test and being able to educate positive patients about immediate isolation against the potential for false negatives, which could send positive patients home thinking they don't have the virus — and thus potentially spreading the disease.

And if a clinician gets a negative result with the rapid isothermal test, but has a strong suspicion the person has COVID or lives in an area with high prevalence, a backup test with a rapid PCR or laboratory-based test should be administered.

"You will miss a certain percentage of people using this rapid isothermal test," she said.

However, Caliendo said, if the only available option is the isothermal test, "you should definitely use it because it's certainly better than not testing at all."

On a positive note, she said, all the varieties of tests have high specificity, so "you're not going to see a lot of false-positive results."

The guidelines back in May didn't make recommendations on rapid tests, she said, because there weren't enough data in the literature.

Caliendo noted that most of the available data were for symptomatic patients, but there are some data that show the amount of virus in the respiratory tract is similar for people with and without symptoms. The panel, therefore, expects that the performance of the various assays would be similar whether or not a person had symptoms.

Testing the Immunocompromised

Hanson said the original recommendation in May was to do molecular testing for asymptomatic people who were awaiting a transplant or were waiting to start immunosuppressive therapy for cancer or an autoimmune disease. Now the current guidelines "make no recommendation for or against screening" in those cases.

Hanson added that the panel feels that patients awaiting bone marrow and solid organ transplants should have the testing because of the high risks that will result if patients have contracted the virus.

But for those with cancer or an autoimmune disease, the panel decided to leave it up to each physician to assess individual risk and determine whether the patient should be tested.

Home Testing

The IDSA guidelines didn't weigh in on home testing because the products are so new and studies so far have included fewer than 200 patients. But Caliendo said they clearly perform better earlier in the disease phase — the first 5-7 days — when the amount of the virus is higher.

Hanson and Caliendo also fielded a question about what the new virus variant, first discovered in the United Kingdom and now spreading to other countries (including the United States) means for diagnostic testing.

"So far we think with the majority of tests that are EUA-authorized, it doesn't look like this new variant should really affect test performance," Hanson said.

The variant has differences in the spike gene, and many of the current tests detect and identify SARS-CoV-2 without the spike gene so they wouldn't be affected, she added.

Caliendo agreed: "I think the vast majority of our tests should be in good shape."

Hanson and Caliendo have disclosed no relevant financial relationships.

Marcia Frellick is a freelance journalist based in Chicago. She has previously written for the Chicago Tribune, Science News and Nurse.com and was an editor at the Chicago Sun-Times, the Cincinnati Enquirer, and the St. Cloud (Minnesota) Times. Follow her on Twitter at @mfrellick

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