Allegheny County and state officials have each begun including antigen test results in daily updates of covid-19 case counts. The new protocol, beginning in late August at both agencies, is approved by Center for Disease Control and Prevention guidelines. But it can still cause confusion when analyzing covid-19 data trends. Here is what you need to know about the different types of covid-19 tests, and what they mean for interpreting case data.
Antigen tests are different from traditional covid-19 tests – but they are both used to indicate an infection.
For months, the covid-19 test of choice was the PCR test, which searches for nucleic acid from the virus that causes covid-19 using mucus samples from inside of the nose.
Dr. Alan Wells, medical director of the UPMC Clinical Laboratories, said these tests are the most reliable: they are able to “amplify” the genetic material of the virus. The PCR test is more sensitive and more reliable as a result, he said. But that also means it’s more technically demanding, resulting in longer wait times for results.
Antigen tests, on the other hand, search for the proteins and lipids in the virus that causes covid-19. The test is faster – with results coming back in as little as 15 minutes – but it is also much less sensitive. Wells said antigen tests cannot amplify the covid-19 genetic material the way PCR tests can; therefore, an individual must have more of the virus present for the antigen test to pick it up.
Antibody tests are completely different.
While PCR and antigen tests are meant to detect a current infection, antibody tests are used to see if an individual had been exposed to covid-19 in the past. The tests search for the presence of antibodies that bind to the virus and prevent it from infecting other cells. Wells said for the most part, antibodies take between 10 and 21 days to become detectable, as the body creates enough of them; that’s why the test can’t be used to diagnose a current infection.
There have already been cases recorded of recovered coronavirus patients becoming infected for a second time, which Wells said is common for respiratory illnesses. These viruses tend to provide less immunity than others, which is why people need to receive such frequent boosters for cold and flu vaccinations. But Wells said the presence of antibodies will help an individual’s immune system work faster if they get covid-19 a second time, resulting in a shorter or less severe illness.
“There is evidence that these antibodies protect a person, they minimize the amount of time you carry and they prevent the spread,” he said. “Herd immunity is never 100%. Herd immunity is never an absolute, but it helps dampen (the virus) down.”
When analyzing state and county data, focus on the cases that have been confirmed, not those that are “probable.”
Both state and local agencies are now reporting antigen tests along with PCR tests in their daily totals.
Maggi Mumma, a Pennsylvania Department of Health spokeswoman, said the change was because of an updated definition for covid-19 from the Council of State and Territorial Epidemiologists – a nonprofit that represents epidemiologists and advocates for public health policies.
“This revised case definition updates criteria for case identification and case classification based on the continued evolution of the covid-19 pandemic,” Mumma said.
The Allegheny County Health Department began using antigen tests to identify probable cases on Aug. 30. Previously, cases with positive antigen tests were counted as probable only if the subject also had covid-19 symptoms or had close contact with a known case.
Antigen tests were authorized for emergency use by the Food and Drug Administration, but Wells said the tests have been known to produce both false positives and false negatives – Ohio Gov. Mike DeWine’s false positive test in August was via an antigen test, for instance.
Some states with travel restrictions will allow visitors if they have tested negative, but won’t accept antigen tests as the testing method, Wells said. The test’s lack of sensitivity leaves more room for error.
That’s why antigen test results should be considered probable cases, not confirmed, Wells said. He said they are best used for younger individuals like college students, who are not at high risk of death by the disease, and positive cases should also be verified later using a PCR test.
“Especially in susceptible people,” Wells said, like those in nursing homes or with compromised immune systems, “I want a sensitive test.”
And when it comes to analyzing the data, monitoring spikes in cases that officials say are likely to occur as schools and colleges reopen, Wells said it is best to only consider cases that have been confirmed. That is the most accurate way to see changes over time without the tallies being inflated with probable infections, he said.
Teghan Simonton is a Tribune-Review staff writer. You can contact Teghan at 724-226-4680, tsimonton@triblive.com or via Twitter .
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