Collecting nasopharyngeal swab (NPS) samples for COVID-19 diagnostic testing poses challenges, including the risk of exposure to healthcare workers and supply chain constraints. Saliva samples are easier to collect but can be mixed with mucus or blood, and some studies have shown that they produce less accurate results. A team of researchers has found that an innovative protocol that processes saliva samples with a bead mill homogenizer before real-time PCR (RT-PCR) testing results in higher sensitivity than NPS samples. Their protocol appears in The Journal of Molecular Diagnostics, published by Elsevier.
“Saliva as a type of sample for COVID-19 testing has been a game-changer in our fight against the pandemic. It has helped us to increase the population’s compliance with the tests as well as to reduce the risk of exposure of healthcare workers during the collection process, ”said lead researcher Ravindra Kolhe, MD, PhD, Department of Pathology , Medical College of Georgia, Augusta University, Augusta, GA, USA.
The study included samples from a hospital and nursing home as well as a drive-thru test site. In the first phase (U protocol), 240 pairs of matched NPS and saliva samples were prospectively tested for SARS-CoV-2 RNA by RT-PCR. In the second phase of the study (SalivaAll), 189 matched pairs, of which 85 had been previously evaluated with the U protocol, were processed in an Omni bead mill homogenizer prior to the RT-PCR test. An additional study was conducted with samples with the U and SalivaAll protocol to determine whether homogenization of the beads would affect the clinical sensitivity of the NPS samples. Finally, a pooling strategy of five samples was evaluated. Twenty positive pools containing one positive sample and four negative samples were treated with Omni bead homogenizer before pooling for SARS-CoV-2 RT-PCR assay and compared to controls.
During Phase I, 28.3% of samples tested positive for SARS-CoV-2 from NPS, saliva, or both. The detection rate was lower in saliva compared to NPS (50.0% vs. 89.7%). During phase II, 50.2% of samples tested positive for SARS-CoV-2 from saliva, NPS, or both. The detection rate was higher in saliva compared to NPS samples (97.8% vs. 78.9%). Of the 85 saliva samples tested with both protocols, the detection rate was 100% for samples tested with SalivaAll and 36.7% with the U.
Dr Kolhe observed that the underlying issues associated with saliva’s inferior sensitivity to RT-PCR tests could be attributed to the gelatinous consistency of the saliva samples, which made it difficult to accurately pipet the samples into the plates. extraction for the extraction of nucleic acids. The addition of the homogenization step gave the saliva samples a uniform viscosity and consistency, which facilitates pipetting for downstream assay.
Dr. Kolhe and his colleagues also successfully validated saliva samples in the five-sample pooling strategy. The results of the pooled tests showed a positive agreement of 95% and the negative agreement was found to be 100%. Pooled testing will be essential for mass surveillance for SARS-CoV-2 as schools reopen, travel and tourism resume, and people return to their offices.
“Surveillance for SARS-CoV-2 will remain a public health need,” said Dr Kolhe. “The use of a non-invasive collection method and easily accessible samples such as saliva will improve screening and surveillance activities and avoid the need for sterile swabs, expensive transport media and the risk of exposure. , and even the need for skilled healthcare workers for sample collection. . “
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