Antigen tests outshine PCR at predicting who can transmit SARS-CoV-2
A new study finds antigen tests for COVID-19 do a better job of predicting which patients are infectious than PCR tests, even though the latter tend to be more sensitive.
That conclusion, which came in a medRxiv paper published Sunday, is the result of work from Becton Dickinson and Co. (NYSE:BDX) and Johns Hopkins University scientists, who correlated the results of antigen tests and RT-PCR tests with readouts from SARS-CoV-2 culture assays. The antigen tests were performed with the BD Veritor System for Rapid Detection of SARS-CoV-2 and the RT-PCR tests with the Lyra SARS-CoV-2 Assay from Quidel Corp. (NASDAQ:QDEL).
The advantage of antigen tests, which include the BinaxNOW COVID-19 test from Abbott Laboratories (NYSE:ABT) that the U.S. government began distributing across the country last week, is that they are faster and less expensive than PCR tests. But their lower sensitivity relative to PCR has been a concern as it necessitates follow-up analysis to confirm a negative result.
The BD and JHU comparison found that although within range, the antigen test’s sensitivity of 96.4% (95% CI: 82.3, 99.4) was numerically lower than the PCR test’s 100% (95% CI: 87.7, 100) — aligning with previous studies suggesting antigen-based diagnostics are less sensitive, but that an antigen-positive result is more likely to reflect viable SARS-CoV-2.
In a tweet about the study’s implications, HHS Assistant Secretary for Health Brett Giroir said, “Bottom line: antigen tests may be better than PCR at detecting who is likely to infect others.”
The study also analyzed the specificity of BD’s Veritor and Quidel’s Lyra assay and found the antigen test had slightly higher specificity than the PCR test (98.7% vs. 95.5%). When 38 RT-PCR-positive samples and 176 RT-PCR-negative samples compared to the results of culture assays, the PCR test yielded no false-negative and 10 false-positive readouts; whereas the antigen test yielded one false-negative and three false-positives.
All samples were collected within seven days of symptom onset; multiple studies have demonstrated an inability to culture SARS-CoV-2 starting at eights days post-symptom onset, even in patients who continue to be PCR-positive.
The paper’s authors wrote that the study’s results could set the stage for shorter periods of self-isolation by infected individuals.
Whether the risk associated with the false-negative readouts is low enough to permit shorter isolation and community-wide application may depend on local incidence rates, with higher infection rates favoring use of the antigen tests.
Another open question is whether the correlation between antigen-positivity and viable SARS-CoV-2 will hold up in infected, asymptomatic individuals.
Separately, NIH’s Rapid Acceleration of Diagnostics (RADx) initiative announced on Tuesday six new contracts, three of which will support development of antigen test technologies.
The new RADx contracts are expected to add 500,000 tests per day to U.S. capacity, totaling $98.4 million for point-of-care and other novel test approaches that provide new modes of sample collection, processing and return of results. With the third round of awards, the RADx portofolio contains 22 companies for a total of $476.4 million in manufacturing expansion contracts.
The contracts were awarded to Ellume USA LLC, Luminostics Inc. and Quanterix Corp. (NASDAQ:QTRX), which are developing antigen-based technologies, and to Flambeau Diagnostics, Ubiquitome and Visby Medical for RNA detection tests.