COVID-19 Testing: PCR or Rapid Antigen Test

  PCR Rapid Antigen Test
What it tests for Molecular material from the virus is magnified through polymerase chain reaction (PCR) to detect very small amounts of RNA Proteins from the surface of the virus, not magnified
Type of sample Nasal swab (at Yale) or saliva Nasal swab
Where it is performed In a lab At home kit or in a clinical setting (called point of care testing)
Speed Typically takes about 24 hours to receive a result, but depends on the lab on other factors About 15 minutes

Very accurate, considered the “gold standard.” Can detect very small amounts of virus early in infection. May also detect left over viral material later in infection  when the person is no longer contagious.

Less ability to detect small amounts of virus. Negative results may be false negatives (showing a negative result when infection is actually present) early in disease when the amount of circulating virus is low. Positive tests are highly reliable.
What a positive result tells you Evidence of active viral infection or, late in disease (14-90 days out), evidence of past infection.

Evidence of active viral infection with sufficient levels of virus to be infectious to others. No need to confirm with another test in most cases. Report a positive rapid antigen test to the university.

What a negative result tells you

No evidence of infection at the time of the test.

Does not rule out infection. It is unlikely that you have enough circulating virus to be infectious at the time of a negative test. Perform a PCR test if available. If a PCR test is not available, repeat the rapid antigen test in 24-48 hours. especially in settings of known exposure or if symptoms present.
When to use it
  • Preferred for most accurate diagnosis.
  • For symptomatic testing when the turnaround time is good.
  • For screening when you do not have symptoms. 
  • For symptomatic testing when a rapid answer is needed (trust a positive, repeat a negative as noted above).
  • For testing immediately prior to an indoor event or travel if you do not have symptoms (a single negative test cannot fully rule out infection, but it is unlikely you are infectious).