The inner workings of a PCR test!
Now that we’ve walked through the science of an antigen test, let’s do the same with a PCR test. This one doesn’t have a visual readout that you can see with your own eyes so it’s a bit more abstract – let’s dive in!
PCR stands for polymerase chain reaction and involves the exponential amplification of very specific pieces of DNA. This is a very useful molecular tool because you can take an incredibly small amount of material and make enough of your target to get a detectable signal. This is what makes the PCR test more sensitive than an antigen test – a smaller amount of virus can be picked up.
Swipe through to get a better understanding of how PCR is used to detect a pathogen and how it can be utilized differently than an antigen test.
Important definition:
– Ct value (cycle threshold value): the PCR cycle at which the target DNA amount reaches a threshold.
– Ct values are inversely proportional to the amount of pathogen starting material.
Interestingly, mutations in a pathogen genome can potentially block primer binding or target signaling making the PCR test fail. In this case, the primers would need to be re-designed to bind to a new region out of the way of the mutation. An example of this is with the Omicron variant of C-19 (also Alpha variant). Omicron contains a mutation in the S-gene that results in S-gene based PCR tests failing. In this way, Omicron can be detected by running it through two differently designed PCR tests (one targeting the S-gene sequence and the other not). In this case if the non-S-gene test is positive and the S-gene test is negative, it is likely Omicron. This is not an efficient way of detecting a variant, but is a fun thing to think through! Genome sequencing is still the gold standard way of quantifying variant prevalence.