July 25, 2020 – FOR IMMEDIATE RELEASE –
Understanding COVID-19 PCR Testing
By Garrett County Joint Information Center Team
From the desk of Dr. Jennifer Corder, Deputy Health Officer for Garrett and Allegany Counties. Diagnosing COVID-19 requires either a PCR test or an antigen detection test. The PCR test is the type of test used for the majority of people diagnosed with COVID-19. The community needs to be aware of the imperfections of these tests. Recently there has been a lot of chatter about so-called “false positive” PCR tests. The likelihood of a false positive PCR for COVID-19 test results is exceptionally low.
PCR tests are fraught with a different problem. They produce an abundance of false negatives. This means people are given news making them think they do not have COVID-19 when in fact they do. You can only imagine how this results in further undetected spread. Anyone who has had one positive PCR test is by definition, a COVID-19 case. It does not matter if the person has symptoms or not.
Usually, there is no need to repeat a PCR test, but some have had their tests repeated. If a repeat test is done and it is negative, one of two things has occurred.
Option one: the person has convalesced in the interval and is no longer shedding the virus and the test is truly negative.
Option two: the person is no longer shedding the virus at detectable levels of the test itself (some tests only detect viral levels at relatively higher concentrations and some tests are more sensitive and detect viral levels at lower levels for example). The test is false negative.
Once you test positive, you isolate for a designated period of time, and a subsequent negative test does not change that recommendation.
False negatives occur frequently. There are people in some hospitals on ventilators who have false negative COVID-19 results! Eventually, when retested, you will find (if you take a sample from deep in the lungs) the test is finally positive. Our testing situation is imperfect, but it’s all we have.
So how do we manage this imperfect situation with regard to testing? We wear masks, distance and wash our hands. We act as if we might have it and we protect ourselves and our families from others who might have it. That’s the state of the world. We are coexisting with COVID-19 indefinitely and balancing risk and benefit. We have all of our eggs in one basket, the vaccine basket. Until an efficacious and safe vaccine becomes available, we need to use the only tools we have in our toolboxes.
Antibody tests are a different topic entirely, and I have not addressed it here.
For more information call the Garrett County COVID-19 hotline at 301-334-7698.