In most of the Covid-19 centres in the country, nasopharyngeal (NP) swabs, which require inserting a long shaft into the nasal cavity to collect a sample from the back of the nose and throat, are being considered the gold standard for collecting a specimen for diagnosis. However, researchers, based on their study have found that sputum testing detects the RNA of the disease causing virus at significantly higher result rates compared to tests based on NP swabs.
In their study published in EBioMedicine, investigators from Brigham and Women’s Hospital conducted a systematic review and meta-analysis, analyzing data from more than 3,000 specimens to compare the three approaches. The team found that sputum testing detected the RNA of the virus that causes COVID-19 at significantly higher rates while oropharyngeal swab testing had lower rates. Regardless of the collection method, the earlier samples were collected after symptoms began, the higher the detection rate.
“The accurate diagnosis of COVID-19 has implications for health care, return-to-work, infection control and public health,†said corresponding author Jonathan Li, 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.â€
Li and his colleagues scoured the literature — both preprints and published papers — for studies that assessed at least two respiratory sampling sites using an NP swab, oropharyngeal swab or sputum. From more than 1,000 studies, they identified 11 that met their criteria. These studies included results from a total of 3,442 respiratory tract specimens.
The team examined how often each collection method produced a positive result. For NP swabs, the rate was 54 percent; for oropharyngeal swabs, 43 percent; for sputum, 71 percent. The rate of viral detection was significantly higher in sputum than either oropharyngeal swabs or NP swabs. 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,†said Li. “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.â€
