Saliva PCR tests may detect disease earlier in the course of an infection
After her children were exposed to COVID, Megan, a woman in Illinois, took them to get tested. A rapid test from the local drugstore might have sufficed, but her son Brennan really did not like the nasal swab—it tickled, it was sometimes painful, and it was just not a fun experience. So Megan took her children to a testing site where Brennan spat into a tube that was then sent off to a lab for processing. The results came back in less than 24 hours.
CovidSHIELD is a polymerase chain reaction test, a highly sensitive molecular assay that involves amplifying genetic material from SARS-CoV-2, the virus that causes COVID, to a detectable level. PCR analysis is usually but not always done in a laboratory. The time lag between taking a COVID test and getting the result is a critical factor in evaluating any diagnostic. Along with overall testing sensitivity, it is part of a broader debate about the use of PCR versus rapid antigen tests. Antigen tests are mainly nasal-swab-based, although researchers are working on an accurate saliva-based version. And they work by testing for the presence of viral antigens—bits of the virus that provoke an immune response—using a specialized testing strip.
But Smith, who was part of a team whose research informed the U.S. Centers for Disease Control and Prevention’s policy on how to use antigen tests, says saliva-based PCR assays have very different use cases than those of antigen tests. “Saliva-based PCR tests are very sensitive, and they can actually pick up infections before you’re infectious,” Smith says. They are as sensitive or even more sensitive than nasal-swab PCR assays.
Once someone is no longer infectious, saliva PCRs may still show up as positive because of their sensitivity. At this point, an antigen test may be more appropriate for determining when someone is no longer infectious, Smith says. “I don’t recommend any PCR [test] for exiting isolation, but antigen tests can be very helpful in that regard,” she adds.
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