Covid-19

Antigen vs. PCR Comparison

May 6, 2021

Heard in the news that rapid antigen tests aren’t sensitive enough compared to PCR tests?

We’re here to clarify some of the misconceptions around that statement and to help you understand the difference between rapid antigen tests and the “gold standard” PCR tests that have been prominent for most of the pandemic.

PCR TESTS

  • Detect RNA (genetic material) of SARS-CoV-2
  • Amplify a lot of copies of the sample so easier to detect that target
  • RNA persists for ~90 days, even when that viral RNA can no longer infect a new host
  • Typically at least a 1-3 day turnaround time
  • Clinical diagnostic

RAPID ANTIGEN TESTS

  • Detect antigen, protein on the outside of SARS-CoV-2
  • Don’t need to make extra copies (amplification) to be able to detect No amplification
  • Antigen persists only so long as virus is able to infect and replicate
  • Typically 15-30-minute turnaround time
  • Tend to be cheaper
  • Currently used for population screening

Why does this difference exist? In a way, in detecting two different targets, these two tests are also serving two different purposes. In looking for a target that is essentially representative of whether or not the virus is “live” and able to infect cells, a rapid antigen test is a tool to determine whether or not you can transmit the virus to other people, spreading the disease. This window is relatively short (within the 10-day window recommended by the CDC to isolate or quarantine if you’ve been infected or exposed). On the other hand, RNA can persist for a relatively long time, well beyond that 10-day window, because of its decay rate. Even if this is in small amounts, the sensitivity of a PCR test means that the residual, “dead” virus can be picked up — and this is well beyond that 10-day window of concern.

Thus, while both of these tests are important and they both come with caveats, a rapid antigen test as an instantaneous snapshot is more helpful to determine whether or not you will be transmitting virus to others and spreading disease. In a sense, the difference in use, the difference in targets, and the difference in the rate that these two targets decay and “disappear” makes it hard to compare the two tests — while both are useful, they serve different purposes.

The Big Picture

  • The CDC requires an isolation period of 10 days if infected with SARS-CoV-2, the virus that causes COVID-19 and a quarantine period of 10 days if exposed to someone infected.
  • Viral RNA — what’s detected by PCR — can hang around LONGER than that 10-day period that someone is deemed to be infectious. This means that a PCR test can sometimes return a positive result beyond this period of interest.
  • However, viral antigens — what’s detected by rapid tests — hang around for a shorter duration that aligns with this isolation/quarantine period
  • While PCR tests are still important and accurate especially for a clinical diagnosis, rapid tests are a helpful tool to figure out if you’re actually at risk of spreading disease
  • This makes them especially useful in a community setting when you’re trying to keep your group of people safe and informed!

CareStartTM COVID-19 Antigen tests are an accurate, American-made option for rapid testing. Learn more about how to purchase and implement serial rapid testing using this EUA-authorized tests.

The CareStart™ COVID-19 Antigen test has not been FDA cleared or approved. This test has been authorized by FDA under an EUA for use by authorized laboratories and at the Point of Care by medical professionals operating under a CLIA Certificate of Waiver, Certificate of Compliance, or Certificate of Accreditation.

The CareStartTM COVID-19 Antigen test is manufactured by Access Bio Inc. for Intrivo. CarestartTM  is a trademark of Access Bio, Inc.