Sputum Testing for COVID-19 More Accurate Than Widely Used Methods

The common complaint about nasopharyngeal COVID-19 testing is how uncomfortable it is to have a six-inch swab pushed into the naval cavity. A new review suggests other downsides, including risks to healthcare workers. The authors question whether sputum testing, which has a higher accuracy rate, might be a better option. Here is more information.

BOSTON – The type of COVID-19 testing being performed in most pharmacies and other locations might not be as accurate as other options.

That’s according to a meta-analysis published in EBioMedicine. Brigham and Women’s Hospital-led researchers found that sputum was more accurate than nasopharyngeal or oropharyngeal swabs.


“Nasopharyngeal swabs are one of the most commonly used methods of respiratory secretion sampling for the detection of SARS-CoV2 viral RNA,” the authors write. “However, the use of nasopharyngeal swabs have a number of drawbacks, including that high-quality swab samples are technically challenging to obtain, nasopharyngeal swabbing increases the risk to healthcare providers due to the frequent induction of reflex sneezing/coughing, and the disruption of the supply of swabs, transport media, and personal protective equipment (PPE).”

The study points out that other approaches to collecting specimens -- including from an oropharyngeal swab and sputum -- have been tested in small studies, but that uncertainty remained as to the best method.

For the study, researchers analyzed data from more than 3,000 specimens to compare the three approaches. Results indicate that sputum testing detected the RNA of the virus that causes COVID-19 at significantly higher rates while oropharyngeal swab testing had lower rates.

"The accurate diagnosis of COVID-19 has implications for health care, return-to-work, infection control and public health," said corresponding author Jonathan Li, MD, a faculty member in the Division of Infectious Diseases at the Brigham. "Our gold standard in and out of the hospital is the nasopharyngeal swab, but there's a lot of confusion about which sampling modality is best and most sensitive. Our study shows that sputum testing resulted in significantly higher rates of SARS-CoV-2 detection and supports the use of this type of testing as a valuable method for the diagnosis and monitoring of COVID-19 patients."

Inclusion criteria for the meta-analysis were studies that assessed at least two respiratory sampling sites -- oropharyngeal swab, nasopharyngeal swab, and sputum -- in participants with COVID-19. From 1,039 total studies, the study identified for inclusion 11 that involved SARS-CoV-2 testing results from a total of 3,442 respiratory tract specimens.

The study team examined how often each collection method produced a positive result. The rate was:

  • 54% for NP swabs;
  • 43% for oropharyngeal swabs, and
  • 71% for sputum.

“The rate of viral detection was significantly higher in sputum than either oropharyngeal swabs or NP swabs,” the authors conclude. “Detection rates were highest within one week of symptom onset for all three tests.”

"When it comes to testing, the earlier the better, as diagnostic accuracy is improved earlier after symptom onset, regardless of the sampling site," Li added. "Unlike antibody testing, it's very rare to have a false positive qPCR test when diagnosing COVID-19 early in the course of the disease using these methods."

The study adds that nasopharyngeal swabs are collected through the nasal cavity, oropharyngeal swabs are collected by inserting a shaft through the mouth and sputum samples are generally collected by having a patient cough deeply to produce and expel phlegm. One drawback is that not all patients are able to produce a sputum sample, with the authors suggesting that a nasopharyngeal swab might be the best collection method in that situation.

 The meta-analysis included only studies conducted on hospitalized individuals, according to researchers, who call for additional studies of patients who are asymptomatic or have mild symptoms. In addition, the current study did not assess alternative testing methods, such as saliva or anterior nasal swabs (taken from the front of the nose).

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