Q&A: Differences between antigen and PCR tests

Barcelona's Hospital Clínic epidemiologist Anna Llupià explains why rapid tests are not as reliable as some think

Taking samples for PCR testing at Bell-lloc School in Girona, September 24, 2020 (by Marina López)
Taking samples for PCR testing at Bell-lloc School in Girona, September 24, 2020 (by Marina López) / ACN

ACN | Barcelona

November 29, 2020 04:43 PM

Anna Llupià is a doctor, specializing in preventative medicine and epidemiology at Hospital Clínic in Barcelona. Although PCR tests have been used for the majority of the pandemic, recently, rapid antigen tests have become more and more common, with a study stating that they could even help to reactivate some social activities. In an interview with Catalan News, Llupià explains the differences between the two forms of coronavirus detection and why she is skeptical about some of the suggested uses for antigen tests, such as reopening venues.

How reliable is each test?
PCR tests are considered the 'gold standard', it is the reference test which you rely on the most. The antigen test has a specificity superior to its own sensibility and this means that you can rely more or less on its positives but you can't rely on its negatives.

In large crowds or large meetings, where people have decided to do an antigen test and afterwards have contact without distancing and so on, there has been spreading [of the virus] despite everyone testing negative because from these [negatives] some positives escape.

How long do you have to wait for a result?
The PCR test takes around two to three hours in a machine but the sample has to reach the lab. After that, it needs to finish off and the result must be uploaded on a platform so that it is available to be passed on to the person [who took the test]. The antigen test is pocket-size, the result comes in 15-20 minutes and you can read it there and then. It also gets uploaded, but you can find the results out straight away.

In what situation could antigen tests be useful?
I think the antigen test can give us advantages in these situations, where we want to go looking for something but we are not sure if we will find anything. Let's take a barbecue for example. You go to this barbecue, you gather these people and do a first round of [antigen] tests on everyone, in which you will probably find something. You give 20 people the test and if you find two or three that are positives maybe then you have to repeat with PCRs.

How do these tests work?
The antigen test finds pieces and elements of this virus such as proteins. The PCR makes copies of the RNA.

How are samples collected?
The PCR as well as the antigen tests have a problem as you can have issues in collecting samples. Sometimes as you collect the sample you do it incorrectly. Both the PCR and antigen are very similar, you have to do a nose smear.