Induced Sputum Better Than Throat Swabs For Detecting COVID-19 Virus?

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes coronavirus disease 2019 (COVID-19), is more readily detected in induced sputum than in throat swabs of patients recovering from COVID-19, according to preliminary research from China.

"To reduce the risk of disease spread, viral RNA tests of induced sputum—not throat swabs —should be assessed as a criterion for releasing COVID-19 patients," suggest Dr. Huanqin Han and colleagues of the Affiliated Hospital of Guangdong Medical University in The Lancet Infectious Diseases.

Detection of SARS-CoV-2 RNA in throat swabs is the most common method of diagnosing COVID-19 in China. But this method has a rate of false-negative results that could allow patients recovering from COVID-19 to meet criteria for discharge from hospital and release from quarantine, fueling spread of the disease, they point out.

Whether testing sputum samples in COVID-19 patients is more sensitive than testing throat swabs is unclear. But investigating this is not straight forward, as most of these patients do not have sputum, especially during the convalescent period.

Dr. Han and colleagues resolved this problem by inducing sputum production in two patients with COVID-19. To induce sputum, the patients inhaled 10 mL of 3% hypertonic saline through a mask with oxygen at a flow rate of 6 L/min for 20 min or until sputum was produced.

The first patient was a 54-year-old man with a 3-day history of fever who was admitted for treatment and quarantine on February 1. On February 3, an approved viral RNA detection kit confirmed mild COVID-19. The patient's fever and symptoms resolved after treatment with oxygen, lopinavir/ritonavir and moxifloxacin.

Beginning February 13, three consecutive throat swabs performed at least 24 hours apart and one anal swab tested negative for viral RNA. One week later, induced sputum was positive for viral RNA.

The second patient was a 42-year-old woman who was admitted on January 27 with a 4-day history of fever. The next day, viral RNA was detected in a throat swab, confirming a mild case of COVID-19. The patient's fever subsided following treatment with oxygen, lopinavir/ritonavir, and moxifloxacin, and her symptoms gradually resolved.

Beginning February 10, three consecutive throat swabs over 3 days and one anal swab tested negative for viral RNA. On February 21, viral RNA was detected in induced sputum.

These results suggest that induced sputum "might be more helpful than throat swabs for the detection of SARS-CoV-2 RNA in convalescent patients," the researchers write.

Dr. Han cautioned in an email to Reuters Health, "We have only reported two cases. Although sputum induction might be more helpful than throat swabs for the detection of SARS-CoV-2 RNA in convalescent patients, the sensitivity and safety of this technology for diagnosing COVID-19 are uncertain, and we are currently conducting a clinical study in this area. I think, to confirm our findings, more patients should be tested using this method to further test its viability for clinical application."

It's worth noting, Dr. Han said, that while sputum induction is a non-invasive method for collecting airway secretions, it is associated with the risk of bronchospasm. "Therefore, professionals need to pay attention to the exclusion of contraindications for patients before using this technology."

—Megan Brooks

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