PCR (polymerase chain reaction) samples can come from several different sites in the patient. The simplest is the nasal swab taken from well inside the nose. The back of the throat is another option.
To find evidence of the virus, researchers use PCR to copy and amplify any segments of viral genetic code found in the sample, which makes it easier to detect. This typically involves adding reagents and enzymes, and raising and lowering the temperature of the mixture, known as thermal cycling.
Antigen testing reveals whether someone has a current infection and could therefore pass Covid-19 on to others. PCR is the gold standard in viral diagnostics.