Swab Testing May Miss Coronavirus Carriers, Posing a Dilemma for Doctors

The Israeli health care system has relied on tests using swabs to diagnose the virus, but the method is reportedly only up to 70% accurate

Ido Efrati
Ido Efrati
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A person is checked for the coronavirus at a drive-through station, Arara, March 30, 2020
A person is checked for the coronavirus at a drive-through station, Arara, March 30, 2020Credit: Rami Shllush
Ido Efrati
Ido Efrati

The Israeli medical system has been relying on swab-sample testing to check patients suspected to have the coronavirus, and making decisions regarding isolation and treatment based on these their results. However, doctors face a dilemma when patients test negative but have clear symptoms of infection.

Results of a study recently published in the Journal of the American Medical Association — based on 1,070 tests on 205 patients — found that swab-sample testing through the nostrils spotted only 63 percent of those infected with COVID-19, and that those whose samples were taken from their throats only identified 32 percent of those infected. Experts suggest that performing both tests increases the accuracy to 70 percent, assuming that the samples are properly obtained.

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Haaretz Weekly Ep. 72

“I am seeing more and more patients with clear clinical symptoms of COVID-19, including people with a consistent background and findings in both lungs but with a negative swab,” said Prof. Dror Mevorach, who heads the coronavirus department at Hadassah University Hospital in Ein Kerem, Jerusalem.

“The reason for that could be swab sampling that isn’t performed correctly, but it’s possible that it’s due to the absence of the virus in the upper airways and its presence in the lower airways, [meaning] becoming infected without excreting the virus. The problem is that patients with clear symptoms from a clinical standpoint, who are deemed negative for the coronavirus at the beginning, can still infect others,” he explained.

Mevorach recounted that this week, two patients came to the emergency room with clear coronavirus symptoms, one of whom had returned from the United States with suspicious medical findings, but their swab test results were negative. In such circumstances, Mevorach and others recommend further investigation. Hadassah Hospital therefore has a special unit for patients suspected of having the coronavirus whose swab tests have come back negative.

Prof. Dror Mevorach, who heads the coronavirus department at Hadassah University Hospital, Jerusalem, April 2, 2014
Prof. Dror Mevorach, who heads the coronavirus department at Hadassah University Hospital, Jerusalem, April 2, 2014Credit: Olivier Fitoussi

Mevorach said from the medical literature on the coronavirus, as well as from conversations with doctors from Italy and the United States, it is apparent that such situations are not getting sufficient attention. In places other than Israel where the coronavirus outbreak is more extensive, coronavirus diagnosis is no longer based on swab sampling, he added.

One diagnostic option that doctors have, called bronchoalveolar testing, involves the insertion of a tube into the lower airways to obtain a sample of lung fluid. According to a published study, this method is deemed 93 percent reliable, but Mevorach said it comes with a greater risk of infecting medical staff.

Another method involves the use of an inhalation device to extract mucus samples from patients’ lower airways. The method poses less of a risk to medical staff if they are properly protected and their presence in the room is limited. Although medical protocols exist for this approach, it is not used enough, Mevorach said.

Swab sampling can also be complemented by a test to determine if patients have antibodies against the coronavirus. “Tests of this kind, when they’re done properly, can be complementary in cases of patients with clear symptoms and a negative swab,” Mevorach said, adding that this is so as long as patients don’t have a compromised immune system.

Testing for the presence of antibodies in such circumstances has not yet been approved, but several companies are working on its development.

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