Rapid Covid-19 tests are better at detecting the omicron variant using nasal swabs rather than throat swabs, according to a recent preprint study published in medRxiv, leading some experts to recommend against using a throat swab to detect the coronavirus.

Study details

For the study, researchers examined Covid-19 test results from four days in January from a community testing site in San Francisco. BinaxNOW rapid Covid-19 tests were used, and their results were later confirmed by PCR tests.

Researchers used the rapid tests as nasal swabs, cheek swabs, throat swabs, or a combination.

The researchers found that the sensitivity of the BinaxNOW test in detecting a PCR-confirmed case of Covid-19 was 74% when using a throat swab alone and 97.7% when using a nasal swab alone. Adding a throat swab to a nasal swab resulted in a slight—less than 5%—increase in detection.

Cheek swabs performed the worst of the group, with just a 9% sensitivity for PCR-confirmed Covid-19 cases with a cycle threshold of <30, which indicates a high viral load.

Overall, the researchers found the BinaxNOW test had a higher sensitivity rate—97.6%—among symptomatic patients, but the sensitivity rate for asymptomatic patients was still relatively high at 89.8%.

Comments

According to the authors of the study, throat swabs "significantly underperformed compared to the standard nasal swab. Even with the prominent clinical feature of pharyngitis in persons with omicron, our data argue[s] against replacing nasal swabs with throat swabs for diagnosis."

Previous research has suggested that omicron may be more detectable in a person's throat than their nose, especially in the early days of infection. For example, a preprint study from South Africa, found that tests using saliva taken from throat swabs were better at picking up omicron than those using nasal swabs. The tests using throat swabs were able to detect 100% of omicron cases while nasal swabs only detected 86% of omicron cases.

However, FDA has cautioned against using throat swabs with at home rapid tests, primarily because the tests were initially authorized as nasal swabs.

"The FDA advises that Covid-19 tests should be used as authorized, including following their instructions for use regarding obtaining the sample for testing," said Jim McKinney, an FDA spokesperson. "The FDA has noted safety concerns regarding self-collection of throat swabs, as they are more complicated than nasal swabs—and if used incorrectly, can cause harm to the patient."

In a series of tweets, Michael Mina, a former epidemiologist at Harvard University and current chief science officer at eMed, noted that "[t]here's a curious fervor in the public domain that to detect omicron 'it requires a throat swab.'"

However, Mina added, "almost every positive case—millions and millions—detected and reported in the U.S. are from nasal swabs."

Evidence could still "tilt toward throat swabs," Mina said, but "[h]onestly we don't know. But we do know that throat swabs are not necessary to detect omicron. The virus will grow exceptionally well in the nose just like the other variants."

Mina added that he can "biologically" support using a throat and nasal swab together because "it can boost sensitivity."

"But don't only use a throat swab," he said. "I do think that's a bad idea." (Walker, MedPage Today, 1/20)