Also called “Nucleic Acid Amplification Testing” (NAAT, or NAT), PCR is a relatively new technology invented in 1983 (its inventor received a Nobel prize). It consists of finding tiny fragments of DNA in specimens and multiplying them thousands of times to be able to identify them. It’s used to diagnose many infections and genetic diseases, and also used legally in paternity cases & criminal investigations.
One main clinical problem with PCR is that it’s such a strong tool that it can sometimes cause misleading, practically false-positive, results. For example, the common nasal swab for Covid-19 uses PCR. But at the tail end of the disease, the test might turn positive if it simply detects fragments of the virus, which are not contagious. See Covid tests.