To PCR or Antigen Test?-that is the question

With many people relying on at home rapid antigen tests to be able to safely gather with family members and attend events, it’s important to know just how far these tests go at helping us assess risk. There is a range of variability in the sensitivities of these rapid home tests especially when it comes to the omicron variant. Sensitivity refers to the ability of a test to accurately give a positive result in someone who is infected with the virus. Numbers for these rapid antigen tests have ranged with some studies showing sensitivity as low as 44% in asymptomatic individuals and an overall sensitivity of 65% and others showing sensitivity of roughly 92% in symptomatic individuals and 78% in asymptomatic individuals. The accuracy of these tests as discussed in the studies does depend on whether one is symptomatic or not (with symptomatic being more accurate) and of course at what point in the course of the illness one is being tested. That being said, there is some utility to rapid antigen tests.

A rapid antigen test detects a protein on one of the spikes of the coronavirus and is less sensitive overall than a PCR test because it takes more viral particles for it to reveal a positive result. A rapid antigen test may be best used if one is trying to determine if still infectious and a risk to others. It is unclear at this point how much viral load one needs to carry in order to be considered infectious, but if the rapid antigen test is positive, the result should be trusted. The timing of when to take the test is important though since you may not have a high enough amount of virus to be detected if you test too early after exposure (which is why PCR would be better early on) so if asymptomatic after a known exposure, testing with a rapid antigen test around day 3-5 and again at day 5-7 since exposure would be more accurate.

A PCR test also known as a reverse transcription-polymerase chain reaction test can pick up small amounts of viral genetic material. The test then amplifies the genetic material, making it a very sensitive test. However, because it reveals positivity even when there’s a small amount of virus, it can actually stay positive for many weeks after one is infected even if one may no longer be “infectious.” An appropriate time to take a PCR test is when one has had a known exposure to someone with COVID-19 or is experiencing symptoms. Since initial studies suggested that the 1-2 days before symptom onset and 2-3 days after symptom onset may be when one is the most infectious, it is important to opt for a PCR test because it may detect the virus earlier than a rapid antigen test, which is not as sensitive. That being said, a recent Japanese study found that viral loads for omicron were highest at days 3-6 after symptom onset, which also calls into question the shortened quarantine guidelines put out by the CDC.

Of course, it’s important to recognize that access to affordable testing is a major obstacle for many. This is why we need to ensure that everyone has access to free testing and delivery of these tests is equitable. For more information about specific rapid tests and their sensitivity, refer to the links below.

https://pubmed.ncbi.nlm.nih.gov/34242764/

https://www.cdc.gov/mmwr/volumes/70/wr/mm7003e3.htm

https://www.nytimes.com/article/at-home-covid-tests-accuracy.html?ref=oembed

https://www.health.harvard.edu/diseases-and-conditions/if-youve-been-exposed-to-the-coronavirus

https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2783099

https://www.niid.go.jp/niid/en/2019-ncov-e/10884-covid19-66-en.html