May 29, 2020 / Matthew Binnicker, Contributor
Dr. Matthew Binnicker, an expert in the diagnosis of infectious disease, explains why someone might still test positive for Covid-19 weeks after they’ve recovered.
To date, the majority of patients with Covid-19 have been diagnosed using a laboratory test called PCR, which detects the virus’ genetic material (i.e., RNA) in clinical samples (e.g., nasal swabs). PCR is a very sensitive laboratory method – meaning it can detect minute amounts of viral RNA – and has been used for nearly 2 decades to diagnose a variety of infectious diseases, including influenza and strep throat. Despite being a rapid and inherently sensitive test, PCR has certain limitations that need to be carefully considered when interpreting the results.
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. From prior experience with other infectious diseases, we know that PCR tests can be positive for days or weeks after a patient has recovered from the illness and is no longer infectious.