COVID-19 tests can detect active or past infections, potential immunity

June 30, 2020

Many people worry that they may have COVID-19, the disease caused by the SARS-CoV-2 virus, or have been infected by SARS-CoV-2 and not known it.

Those who have recovered from the virus likely wonder whether they are protected from getting it again.

Testing can provide answers.

There are currently two broad categories for COVID-19 tests: one is to check for active infection (i.e., the presence of virus) and another is to check for previous infection.

Tests to diagnose an active COVID-19 infection

If someone has COVID-19 symptoms, a viral test can reveal if they are currently infected with the virus. In a viral test, a sample of fluid is collected from the respiratory system (such as swabs of the inside of the nose or throat, or saliva) to confirm the presence of the SARS-CoV-2 virus.

  • Molecular (PCR) test: A polymerase chain reaction (PCR) test is a molecular diagnostic test that detects genetic material from the virus. PCR tests are very sensitive and specific for the presence of SARS-CoV-2. There are several different types of PCR tests currently available, some of which can generate a result in as fast as 15 minutes.
  • Antigen test: An antigen test detects viral proteins found on or within the virus. Antigen tests are very specific for SARS-Cov-2, but are not as sensitive as molecular (PCR) tests. A positive antigen test, just like a positive molecular test, indicates the patient is infected with the virus and presumed to be contagious. However, since antigen tests are known to be less sensitive than molecular tests, negative antigen test results should be treated as presumptive and confirmed with a molecular (PCR) test, if necessary.

For patients with a doctor’s order from an Edward-Elmhurst Health (EEH) physician, drive-up COVID-19 molecular (PCR) testing is available in the EEH Corporate Center parking lot (4201 Winfield Road, Warrenville). Patients should ask their doctor whether a drive-up test is appropriate for them.

The FDA has authorized the use of certain at-home COVID-19 test kits in which people collect their own samples (either a nasal swab or saliva) at home and mail it to a lab for processing. Home tests are available only to individuals with a physician order and are not currently offered by Edward-Elmhurst Health.

Test to detect past COVID-19 infections

When someone is infected with a virus, their body’s immune system produces proteins called antibodies to fight the infection. An antibody (or serology) test checks a sample of blood (usually by a blood draw) for antibodies specific to SARS-CoV-2 to determine if the person has been infected with this virus and has developed an immune response.

Since it can take several days to three weeks (or longer) for the body to develop antibodies after infection, an antibody test does not indicate whether someone is currently infected with SARS-Cov-2; a positive antibody test only reveals that someone has been infected by the virus at some point in the past. It is important to remember that people can have a positive antibody test and still be contagious to others if the infection occurred only several days prior to testing.

In addition, research is underway to determine if having antibodies can protect people from reinfection and, if so, what level of antibodies is needed for immunity and how long immunity might last. For this reason, individuals with a positive antibody test should not assume that they cannot be reinfected by SARS-CoV-2 and should continue practicing social distancing and all the other recommendations for preventing disease spread. For now, the antibody test helps experts better understand the prevalence of disease in our community.

This summer, the Centers for Disease Control and Prevention (CDC) will launch an 18-month nationwide COVID-19 antibody study to better understand the immune response to COVID-19 and whether it diminishes over time.

The antibody test can also help identify individuals who may be eligible to donate a part of their blood called convalescent plasma, which contains antibodies against the virus, and may help treat those who are seriously ill from the virus.

Edward-Elmhurst Health, in conjunction with Versiti, Inc., is looking for patients who have recovered from COVID-19 and would be interested in donating plasma. The donor requirements for donating plasma are similar to those for donating blood, but also include documentation of a previous positive COVID-19 molecular (PCR) test and a clinical history of no symptoms for at least 14 days. The process for donating plasma is very similar to donating blood.

Anyone interested in donating plasma to help treat others diagnosed with the virus should contact Versiti at

For updates on COVID-19, check