The sensitivity of DART was calculated for each day. DART sensitivity was 96.3 percent (26 of 27) within days zero to three of symptoms. Eleven participants tested positive on day one or two. One was presymptomatic the day of their initial DART positive result. One infection was detected by qRT-PCR one day before DART. For one positive participant, DART detected infection one day before qRT-PCR did.

There are a number of issues with the report, said Dr. Otto Yang, a professor of medicine in the division of infectious diseases and of microbiology, immunology and molecular genetics at the David Geffen School of Medicine at the University of California, Los Angeles.

First, Dr. Yang said, “they apparently use a home-brewed RT-PCR kit, and not an FDA-approved commercial kit. Is their test as good as the commercial tests? What was their limit of detection? It’s true that RT-PCR is the gold standard, but not all RT-PCR tests are equal.”

Beyond that, “the overall number of patients is only 15,” Dr. Yang said in an email. “By any standard, that’s pretty small. They inflate the number by mentioning the number of swabs since there were multiple swabs per patient, but the bottom line is that 15 is quite few.”

With so few participants, “the sensitivity and specificity numbers are hard to trust; they are doing these per swab rather than per patient,” Dr. Yang said. “It would be easy to get better specificity numbers by running more samples on negatives.”

Moreover, “many people are asymptomatic–especially if vaccinated–or shed virus for a couple of days before showing symptoms,” Dr. Yang said. “How does the test compare to PCR in those situations? From the standpoint of public health, this is critical. Confirming a diagnosis in someone who is already symptomatic is useful, but not as useful as finding infection in people without symptoms.”

The paper is in line with the current knowledge about antigen testing for COVID-19, said Dr. Heba Mostafa, an assistant professor of pathology at the JHU School of Medicine.

“Rapid antigen testing has lower sensitivity (can cause false negatives) compared to the gold standard nucleic acid amplification methods (for example, PCR), yet highly specific, Dr. Mostafa said in an email. “The sensitivity of antigen testing is largely highest when viral loads are high and within the first few days after symptoms onset.”

“Even though the gold standard for diagnosis so far remains nucleic acid amplification, as the authors mentioned, the cost could be a limitation as well as turnaround time in certain locations where molecular methods are not readily available,” Dr. Mostafa said. “In these situations, antigen testing could be helpful, especially if used frequently for diagnosis and quarantine. The impact of frequent antigen testing on reducing the transmission of SARS-CoV-2 has been proposed but to the best of my knowledge was not experimentally shown yet.”