PCR vs AG tests for COVID-19

There are two types of COVID tests, a PCR test and an Antigen test. Both tests are available at Habif.

The PCR test measures genetic material from the virus and is a very sensitive test for detecting COVID-19. This is the test that is available at Habif and Barnes Jewish Hospital. It has the potential to remain positive for up to 90 days even though the virus particles may no longer be alive or have the potential to infect others. Hence, we do not recommend retesting with a PCR test within 90 days of a positive result unless there are mitigating circumstances.

The Antigen test looks for a protein on the surface of the virus. One needs more virus to be present in the sample to pick up this protein in the test and have a positive result.  It is a rough estimate of infectivity. Generally, one is less infectious if their antigen test is negative, and the antigen test usually reverts to negative within 5-14 days after infection.

How well do antigen tests work?
In general, they may be able to catch most, but not all, cases of COVID-19 that are contagious at the time of the test. They are not as sensitive as PCR tests, which means that some people can have COVID-19 with a negative antigen test (i.e. a false negative). Antigen tests are more accurate when symptoms are present and may miss more than 50% of asymptomatic cases and 20% of symptomatic cases.

What are the best uses of antigen tests when I do not have any symptoms?
Antigen tests can play an important supportive role in preventing the spread of COVID-19.. Note that a negative test result does NOT rule out infection, but it does make it less likely. If you have symptoms, you should not attend any gathering until you have had a PCR test. If you don’t have symptoms, it is still important to consider taking extra precautions such as masking while indoors when around people who could be at high risk of serious COVID-19 complications. When community transmission levels are high, a positive test likely indicates true infection and should be treated as such. Antigen tests can also be useful to try to detect early infection in those who have had a known close-contact exposure. A test taken 3-5 days after exposure will sometimes detect infection before symptoms are noticed.

What are the best uses of antigen tests when I do have symptoms?
If you have symptoms that could be due to COVID-19, an antigen test may be able to quickly confirm that you have infection. A positive test in that setting suggests that you have the infection and that you can pass it to others. You should treat a positive test as a true result and stay home while contacting Student or Occupational Health. A negative antigen test does NOT rule out infection, and you must be tested with the more sensitive PCR test before returning to work or school. View video.
How can I obtain a COVID-19 test?

Antigen tests are available in the lobby at Habif Monday,Tuesday, Thursday and Friday: 8 a.m.-5 p.m, Wednesday: 10 a.m.-5-pm to pick up at your convenience.

PCR tests will be ordered for symptomatic students (and in other select circumstances such as cluster investigations). If you have concerning symptoms, call the COVID call center at 314-362-5056 or contact Habif Health and Wellness Center to arrange for testing.