By Joanna Storey, MD
In the early days of the covid-19 pandemic, coronavirus tests were scarce and results were slow to return. Fortunately, that situation has improved. At Children’s Medical Group, we currently offer three types of coronavirus tests. If your child is sick or has been exposed to coronavirus, please contact us so that we can discuss the best timing and method for testing and evaluation.
Antigen testing is performed on a swab of a patient’s nose. It checks for the presence of proteins that exist on the surface of the coronavirus. The advantage of this test is speed—an in-office result takes 15 minutes. If this test reads positive for coronavirus infection, it is very likely accurate. However, if it reads negative, there is about a 15% false-negative rate, meaning that a small number of patients may be actively infected even though they have a negative antigen result. Because of the possibility of false negatives, if a patient has a high risk for coronavirus infection, we may decide to follow up a negative antigen test result with a PCR test.
PCR testing for coronavirus is also performed on a sample obtained from a nasal swab. This test uses molecular technology to search for the genetic signature of the coronavirus. Its advantage is accuracy—it is very uncommon to get false-negative or false-positive results from a PCR test. Unfortunately, there is a time delay for results. The sample must be processed at an outside testing facility which typically takes 1-3 days. So that we can get the fastest result possible, CMG uses three different external labs for PCR testing.
Antibody testing for coronavirus is performed on a patient’s blood sample with next-day results. This test looks for evidence of an immune system response to coronavirus infection and may indicate whether or not a patient has had a covid-19 infection in the past. From a practical standpoint, however, this test has some drawbacks. First, it cannot distinguish between current and past infection. Therefore, it is possible for a healthy patient who tests positive for coronavirus antibody to have an asymptomatic active infection and still be contagious to others. Also, coronavirus antibody levels appear to decrease over time. This means that a patient who had coronavirus infection six months ago could have undetectable antibody levels today. Lastly, it is not known if having detectable antibodies means that a patient cannot be infected again. Reinfection with coronavirus seems uncommon, but it has been documented. For these reasons, public health experts caution against using a positive antibody test as evidence of immunity.
Please get in touch with the CMG health care team if you have additional questions about coronavirus testing.