Allegheny County and state officials have begun including antigen test results in daily updates of covid-19 case counts. The new protocol, starting from the end of August at both agencies, is approved by the guidelines of the Center for Disease Control and Prevention. But it can still cause confusion when analyzing trends in covid-19 data. Here’s what you need to know about the different types of covid-19 tests and what they mean to interpret the case data.
Antigen tests are different from traditional covid-1
For months, the covid-19 test of choice has been the PCR test, which looks for nucleic acid from the virus that causes covid-19 using mucus samples from inside the nose.
Dr Alan Wells, medical director of UPMC clinical laboratories, said these tests are the most reliable: they are capable of “amplifying” the genetic material of the virus. The PCR test is consequently more sensitive and more reliable, he said. But that also means it’s technically more demanding, resulting in longer waiting times for results.
Antigenic tests, on the other hand, look for proteins and lipids in the virus that causes covid-19. The test is faster – with results returning in just 15 minutes – but it is also much less sensitive. Wells said antigenic tests cannot amplify 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 an ongoing infection, antibody tests are used to see if an individual has been exposed to covid-19 in the past. The tests look for the presence of antibodies that bind to the virus and prevent it from infecting other cells. Wells said that for most antibodies take 10 to 21 days to become detectable, as the body creates enough of them; that is why the test cannot be used to diagnose an ongoing infection.
Cases have already been recorded of coronavirus patients being cured who have been infected for the second time, which Wells says is common for respiratory diseases. These viruses tend to provide less immunity than others, which is why people need to be given such frequent boosters for cold and flu vaccinations. But Wells said the presence of antibodies will help an individual’s immune system function faster if they get covid-19 a second time, resulting in shorter or less severe illness.
“There is evidence that these antibodies protect a person, minimize the amount of time they carry and prevent their spread,” he said. “Herd immunity is never 100%. Herd immunity is never absolute, but it helps dampen (the virus). “
When analyzing state and county data, focus on cases that have been confirmed, not those that are “likely.”
Both state and local agencies are now reporting antigen tests along with PCR tests in their daily totals.
Maggi Mumma, a spokesperson for the Pennsylvania Department of Health, said the change is due to an updated definition of covid-19 by the State Council and Territorial Epidemiologists, a nonprofit that represents epidemiologists and policy advocates for public health.
“This revised case definition updates the criteria for identifying and classifying cases based on the continuing 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 considered likely only if the subject also had symptoms of covid-19 or had close contact with a known case.
The antigen tests have been cleared for emergency use by the Food and Drug Administration, but Wells said the tests are known to produce both false positives and false negatives: Ohio Governor Mike DeWine’s false positive test in August is was done via an antigen test, for example.
Some states with travel restrictions will allow visitors if they test negative, but will not accept antigen tests as a test method, Wells said. The lack of test sensitivity leaves more room for error.
That’s why the antigen test results should be considered probable, unconfirmed cases, Wells said. He said they’re best used for younger individuals such as college students, who aren’t at high risk of dying from the disease, and positive cases should be checked 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 officials say could occur with the reopening of schools and colleges, Wells said it’s best to only look at cases that have been confirmed. This is the most accurate way to see changes over time without the counts being inflated by probable infections, he said.
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