The COVID-19 pandemic has spurred a considerable amount of testing innovation, including more extensive use of saliva as a medium for detection of pathogens. The U.S. FDA has announced an emergency use authorization (EUA) to Yale School of Public Health for the Salivadirect testing protocol that requires no extraction of the SARS-CoV-2 virus’s RNA, a feature that Assistant Secretary for Health Brett Giroir said is “yet another testing innovation game changer that will reduce demand for scarce testing resources.”
The Yale test is not the first COVID-19 molecular diagnostic to win an EUA with saliva as the test medium. The agency announced May 8 that it had authorized the lab-developed test (LDT) by Rutgers Clinical Genomics Laboratory to process saliva using a self-collection kit designated for this purpose. The Rutgers test was at that time the only saliva-based diagnostic test to be authorized under the EUA program for the pandemic, although it was decreed that the test would be available only by prescription.
Performance said to be similar to nasopharyngeal swabs
Most EUA press releases by the FDA feature a statement by the agency’s commissioner, Stephen Hahn, or a center director, but the sign-off by Giroir would seem to highlight the agency’s appraisal of the importance of this development. Yale published a statement explaining that the test is “simpler, less expensive and less invasive” than other media for molecular diagnostics for the pandemic, such as nasopharyngeal (NP) swabs. Yale said the test is highly sensitive and performs similarly to a molecular test using NP swabs.
Yale said also the testing protocol can be scaled up for use in other labs across the nation “in the coming weeks,” and perhaps in other nations as well. The Salivadirect has been validated with reagents and instruments from multiple vendors, which may help sidestep some of the supply chain issues seen with some testing methods and instruments. The expectation is that labs will charge only about $10 per sample, an estimate based on the use of relatively inexpensive reagents. Previous studies have suggested that the SARS-CoV-2 virus is stable in saliva for “prolonged” periods of time, but Yale offered no comment as to whether, or to what extent, this approach might compress turn-around times from clinical labs.
The FDA statement indicated that any sterile container would suffice for the Yale testing approach, adding that the elimination of the extraction kit used for many other COVID-19 tests will avoid at least one bottleneck. The test requires no proprietary equipment, while the saliva must be treated with proteinase K, then subjected to a heat deactivation step.
The test is based on a study of its use on National Basketball Association players, which Yale said in a June 22 statement was designed to evaluate whether the test would be accurate “at least 90% of the time.” The target date for the EUA was mid-July, and the fact that NBA players were on court without masks is said to have presented the Yale team with “a unique opportunity for a comparison study.”
The Yale approach was briefly described in the now-widely known preprint server Medrxiv, which characterizes the test as a surveillance instrument. A companion piece offering more information lists the instruments and supplies needed to employ the Yale protocol, describing the total reagent cost as possibly as little as $1.29. This approach has been evaluated with three sets of controls and six primers and probes, although the Yale team indicated it will continue to beef up the testing capacity for this approach.
Some of the emphasis here is on robotic automation and on pooling, but they also hope to replace the PCR processing step with an alternative, including recombinase polymerase amplification (RPA). RPA can detect RNA as well as DNA with the help of a reverse transcriptase enzyme, and eliminates the need for a thermal cycler. Another possible isothermal approach cited by the authors is loop-mediated isothermal amplification (LAMP), which when paired with photometry can provide visual confirmation of the target molecule by the naked eye.
Gottlieb had described Salivadirect as a screening test
Former FDA commissioner Scott Gottlieb said in an Aug. 8 tweet that the Salivadirect could be the first test used for asymptomatic screening, although the EUA describes it as a diagnostic to be used for those suspected of infection. Gottlieb added that this protocol “may be the lowest cost test to hit market with ability to be pooled,” adding that the testing process can be completed in two hours. However, he tweeted on Aug. 16 that there is a need for fast and inexpensive lateral flow tests using strips to detect the pathogen. These test types “are popular in other countries, but seldom used in U.S. because of a bias toward lab testing. We must get testing to consumers,” he said.
That view regarding lateral flow tests was echoed by Michael Mina, the Harvard professor who held forth earlier this month on the need for inexpensive testing. Mina said in an Aug. 17 series of tweets that the Salivadirect is not a rapid test, pegging the lab processing turn-around at three hours. He pointed out that much of the cost of lab-based PCR tests is not the reagents, but described the Yale protocol as “a wonderful and hugely needed addition to the arsenal of laboratory-based diagnostics.” Still, he said the new EUA “is not going to be a major game changer,” in part because of skepticism that saliva is a reliable medium for high-throughput processing.