NEW YORK – A study published on Wednesday suggests that results from molecular diagnostic tests using self-collected nasal swabs are comparable to clinician-collected nasopharyngeal swabs.
The results raise the possibility that swab samples collected by potential COVID-19 patients at home could help minimize community exposures and increase access to testing without using up precious personal protective equipment.
As described in a study in the Journal of the American Medical Association, researchers at the University of Washington selected study participants from among symptomatic outpatients who were seen at primary care clinics, emergency departments, and drive-through testing sites. The primary care and ED subjects with positive NP swab results had a self-swab kit delivered to their home within hours of testing positive, while the drive-through enrollees were given a kit to take home.
The self-collection kits contained Copan flocked mid-nasal swabs, Becton Dickinson Universal Transport Media, and written instructions on how to perform a self-collected nasal swab, according to the study. The swabs were returned to the lab either by courier the same day or by US Postal Service, where they were kept refrigerated until molecular diagnostic testing.
Specifically, the UW protocol entailed extraction using the high-throughput Roche Magna Pure 96 instrument and the Viral NA Small Volume Kit. One-step RT-PCR was performed with Thermo Fisher Scientific's AgPath-ID system master mix on the company's Real-Time ABI 7500 instrument. The RT-qPCR used US Centers for Disease Control and Prevention probes for the SARS-CoV-2 nucleocapsid (N) genes N1 and N2.
The UW team defined true positive results as those that were positive for both types of sample collection. False negative results were considered to be those that were positive by clinician-collected NP but negative by self-collected nasal swab.
Overall, the researchers found that home-collected nasal swabs had a sensitivity of 80 percent and a specificity of approximately 98 percent.
Home swab collection was done about four days after symptom onset for the 28 true positive samples, but about six days after symptom onset for the seven false negative samples. Nevertheless, the UW group found that the cycle thresholds of the two types of sample were positively correlated. The Ct is considered an indicator of viral load, and a positive correlation was seen even though the self-collected samples were obtained, on average, one day after clinician-collected ones, and were shipped at ambient temperatures.
Although the sensitivity of SARS-CoV-2 detection using home self-collected samples in this study was lower than previous evaluations of at-home influenza testing — one meta-analysis published last year showed pooled sensitivity of 87 percent and specificity of 99 percent for home-collected flu testing — the authors noted that the false-negative results tended to occur in samples with Ct values higher than 33, equating to low initial viral loads.
Of the 185 participants in the study, 158 were healthcare workers, which the authors said suggested could limit the generalizability of the results. Healthcare worker might, for example, be better at reading instructions or be more familiar with how to take samples for diagnostic testing.
Overall, the authors concluded that at-home sample self-collection "is safe and scalable in the pandemic setting, permitting widespread testing of symptomatic participants early in illness [with] the potential for prompt self-isolation and contract tracing."
At-home sample collection and self-collection methods are increasingly gaining acceptance during the SARS-CoV-2 pandemic.
The US Food and Drug Administration created a template for developers seeking Emergency Use Authorization dedicated to at-home self-collection and the agency has already authorized tests from Laboratory Corporation of America, Everlywell, Rutgers University, Quest Diagnostics, and LetsGetChecked that allow for the use at-home collection of nasal swabs. A self-collected saliva-based test in development by researchers at Yale University is currently being evaluated in collaboration with the National Basketball League.
Meanwhile, an at-home SARS-CoV-2 sampling study that had grown out of a UW-supported influenza home-collection project was halted by the FDA earlier this year until the program could get an EUA, a move that was widely criticized by experts. The Greater Seattle Coronavirus Assessment Network, or SCAN, program was re-launched in June and is now mailing sample self-collection kits to local residents.
In a New England Journal of Medicine study published online in May, preliminary data showed SCAN surveillance could help with early detection of community spread. Many of the authors on the NEJM SCAN study are also authors on the JAMA study, although SCAN is not explicitly referenced therein.