Opinion: Some coronavirus patients test positive for weeks. Interpret those results with caution

One of those key limitations of PCR is its inability to determine whether a patient is infectious, or not. This is because the test is designed to detect the virus' RNA, which is generally present when a virus is causing an active infection. However, RNA can also be present, and therefore, detected by PCR after a virus has broken down (i.e., become non-infectious) and released its genome into host cells or body fluids.