How do PCR and rapid antigen tests tell whether you are infected?

PCR and rapid antigen tests look for different things. A PCR test searches for a virus's genetic code and copies it many times until it is easy to detect; a rapid antigen test looks for a protein on the outside of the virus and shows a stripe if enough of it is present. For a virus such as SARS-CoV-2, those are the two usual routes: the inner RNA, or pieces of protein on the outside.

PCR, short for polymerase chain reaction, works like a photocopier set to double. In each cycle the DNA is heated so its two strands separate, then cooled so short matching pieces called primers attach, and an enzyme builds new strands. Every cycle doubles the amount, so after 30 cycles a single copy can become up to a billion. Many viruses, including SARS-CoV-2, carry RNA rather than DNA, so the test first converts it into DNA. The full process generally takes a few hours.

A rapid antigen test skips the copying. Antibodies stuck to a dye grab any virus particles in the sample, and the mix creeps along a paper strip until it reaches a line of antibodies that catch the virus and the dye together. If enough virus is there, a line appears, usually within 5 to 30 minutes. The price of the speed is sensitivity: these are screening tests, and a positive is generally confirmed by PCR.