Could sputum tests for Covid-19 provide better results than nasal swabs?

  • Currently, a Covid-19 test is performed by taking a sample from the nasal cavity
  • This method is uncomfortable and carries the risk of a false negative
  • In a review, researchers explored the option of a test using a sputum sample 

As Covid-19 is gaining a foothold all over the world, experts maintain that early and accurate detection is the best method to help slow down the spread, especially as some European countries are approaching the so-called “second wave”.

The problem with nasal swab tests

Currently, R- PCR tests are used, where a viral sample is isolated from material gathered from a nasal or throat swab. Even though this is the standard procedure, the procedure is often uncomfortable and requires staff clothed in PPE to perform these tests, increasing their risk of contracting Covid-19, especially where PPE is in short supply.

The accuracy of these tests has been questioned in previous research – a previous Health24 article mentions a study where the occurrence of false-negative results was one in five. These false-negative results could exacerbate the spread of Covid-19, as people who are under the false impression that they don’t carry the virus can potentially infect those at risk of severe Covid-19.

New research on sputum tests

A new study, published in EBioMedicine, conducted a systematic review of more than 3 000 specimens to compare three testing approaches. They found that the virus was more likely to be detected in sputum tests than through nasal or oropharyngeal swabs.

Another thing that the researchers found, was that the earlier the samples were collected after the appearance of symptoms, the higher the detection rate.

"The accurate diagnosis of Covid-19 has implications for healthcare, return-to-work, infection control and public health," stated corresponding author Jonathan Li, MD, a faculty member in the Division of Infectious Diseases at the Brigham, in a press release.

"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."

Sputum tests had highest detection rate

All available literature was compared and reviewed to obtain results for three testing methods:

  • Nasal swabs had a detection rate of 54%.
  • Throat (oropharyngeal) swabs had a detection rate of 43%.
  • Sputum samples had a detection rate of 71%.
  • Detection rates were highest within one week of symptom onset for all three methods.

"When it comes to testing, the earlier the better, as diagnostic accuracy is improved earlier after symptom onset, regardless of the sampling site," Li stated in the release. "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."

Sputum tests are usually collected by having a patient cough deeply to produce phlegm, which is used as a sample. There are some caveats to the sputum test – not all patients may be able to present a sputum sample. The study also excluded testing methods from saliva and anterior nasal swabs.

The researchers are currently working on a project to create a resource for researchers to help develop the most accurate detection method.

"The holy grail will be to find a test that is readily acceptable by patients, easy to collect, and highly sensitive," stated Li.

READ | Scientists warn of false-negative results of Covid-19 tests: What are the implications for SA?

READ | Latest on Covid-19 testing in SA: Public sector backlogs more than 6 days – an expert tells us more

READ | Covid-19: Call to reduce 'unnecessary' screening and testing in SA - and to target areas of concern

Image credit: Getty Images

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