While rapid viral protein tests have been criticized for their low sensitivity compared with viral RNA tests, more epidemiologists now believe antigen tests can still be a helpful public health tool -- as they may indicate who is most contagious at any given moment.
That could have implications for slowing the spread of COVID-19, advocates say.
"I don't care if someone was infectious 2 weeks ago. I want to know if the person I'm testing today needs isolation today," Michael Mina, MD, PhD, of Harvard, said on an episode of CNN's podcast Coronavirus: Fact vs Fiction. "That's where these tests really shine. They will be positive when somebody is contagious, for the most part."
Evidence is starting to grow that the tests are good at detecting people with active infection. In a study published last month in the CDC's Morbidity & Mortality Weekly Report, Abbott's BinaxNOW rapid antigen test had a sensitivity of just 64.2% for symptomatic disease and 35.8% for asymptomatic disease when compared with PCR.
But Mina -- who has been a vocal advocate for rapid testing, with his website RapidTests.org and publications in Science and the New England Journal of Medicine -- says PCR is the wrong comparator.
When compared with samples that grew virus when cultured, the BinaxNOW's sensitivity rose to 92.6% for symptomatic disease and 78.6% for asymptomatic disease.
"It's doing exactly its job," finding people with high viral loads who are most likely to be actively spreading the disease, Mina said on the podcast.
Authors of the MMWR report came to a similar conclusion: "Despite their reduced sensitivity to detect infection compared with real-time RT-PCR, antigen tests might be particularly useful when real-time RT-PCR tests are not readily available or have prolonged turnaround times. Persons who know their positive test result within 15 to 30 minutes can isolate sooner, and contact tracing can be initiated sooner and be more effective than if a test result is returned days later."
Results from a study performed in San Francisco's Mission district, published at first as a medRxiv preprint and subsequently in the Journal of Infectious Diseases, found that BinaxNOW had a sensitivity of 93.3% when identifying infection measured at a PCR cycle threshold (Ct) cutoff under 30, suggesting a high viral load. Without a Ct cutoff, sensitivity fell to 57.7%.
There Are Drawbacks, However
Mina admits that there's a window of about 5 to 10 hours early in infection where antigen tests can miss people who may be contagious.
Authors of the Journal of Infectious Diseases paper noted the same: "During the early stages of infection, viral load may be too low to detect by direct antigen assays."
But large testing regimens would still capture a swath of those who are infectious, the researchers said, and the "inherent lower sensitivity may be offset by faster turn-around and higher frequency of testing, with overall lower cost, relative to RT-PCR methods."
A public health screening program -- even with relatively inexpensive tests like the $5 BinaxNOW -- would require resources to be effective.
The Biden administration recently signed a $232-million deal with an Australian company to boost production of its self-contained COVID-19 antigen test Ellume, the only one with FDA authorization that doesn't require a prescription.
Public health departments, hospitals, and governments would then need to spend money to purchase and distribute them, and on surveillance programs.
Christina Wojewoda, MD, vice chair of the College of American Pathologists' microbiology committee and a pathologist at the University of Vermont Medical Center, said that could detract from other important efforts like vaccination.
"I would much rather focus now on vaccination efforts and on prevention -- getting people to wear personal protective equipment and socially distance," Wojewoda told MedPage Today. "Rather than trying to test everyone all the time to find infections, we should focus on not letting people get infected in the first place."
Wojewoda is also concerned about equity with home-based screening programs, as well as compliance.
"We're relying on people to sample themselves appropriately, to do the testing appropriately, and to interpret the test appropriately," she said. "If someone has to go to work to earn money to feed their kids, are they really going to look at that positive test result and say, 'Okay, I just won't get paid today'?"
"We can't even get people to wear masks correctly," she added.
Finally, Wojewoda noted that the FDA is considering asking antigen test developers about their plans to ensure their tests still detect the novel variants that have arisen. These variants typically affect the spike protein of the virus, which these tests detect.
During a phone briefing with test sponsors on Jan. 29, Timothy Stenzel, MD, PhD, director of the FDA's diagnostics office, said the agency will "in all likelihood" be updating the emergency authorization template, "asking developers what their thoughts are about how they can monitor for the impact of variants."
Still, dozens of respected epidemiologists, public health, and infectious disease experts have signed onto an open letter from Mina's group insisting that when "used appropriately, these tests could curb outbreaks of the virus by reducing transmission."
"They provide a critical layer of protection to supplement masks, social distancing, and other precautions," the letter states. "They can help bridge the gap until broad vaccination is achieved."
Last Updated February 03, 2021
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