When testing for an active COVID infection -- meaning the person is still contagious -- the administrator swabs the nose, back of the nose or saliva to test. The accuracy of the test will depend on how well that sample was collected, along with what test is actually being used. The best test is called a PCR test, which uses a lab technique called polymerase chain reaction (PCR). It is the most sensitive COVID-19 test available and helps tell how infectious a person actually is. See how the PCR test works in the video report above.