The antigen test will detect the virus before it has been clared by the immune system. Essentially it tests whether someone has the virus at the time of sampling. The laboratory test detects the genetic material (called RNA) of the SARS-CoV-2 virus. A sample is collected from the deep nasal area using a swap.
The swab is sent to the laboratory where a technique called Reverse Transcription Quantitative Polymerase Chain Reaction (RT-qPCR or sometimes shortened to PCR) is used to confirm the presence of the virus.