Complex sentence: Unlike antibody tests, PCR tests look for the presence of the virus by using the polymerase chain reaction to amplify even very tiny traces of viral RNA in the bloodstream, to produce measurable amounts of viral RNA which may then be tested for directly.
Split into simpler sentences: Unlike antibody tests, PCR tests look for the presence of the virus directly. They use the polymerase chain reaction to amplify even very tiny traces of viral RNA in the bloodstream, to produce measurable amounts of viral RNA which may then be tested for directly.