There are now multiple types of COVID-19 tests available, including Polymerase Chain Reaction (PCR) and serology (antibody testing). Both test types are becoming increasingly available in our communities. The PCR test is a diagnostic test that looks for the presence of viral nucleic acids and tells if a person is currently infected with the virus. A new diagnostic test, known as an antigen test, was just approved by the Food and Drug Administration (FDA). This test looks for specific parts of the virus, and can also tell you if a person is currently infected with the virus. Serologies check a person’s blood for the presence of antibodies. These antibodies are produced when someone has been infected with a bacteria or a virus. A positive result from a serology test indicates that a person was previously infected with the virus. In some cases, serology tests can also indicate if someone is currently infected with the virus.
There are different types of antibodies that serology tests look for. These include Immunoglobulin M (IgM) and Immunoglobulin G (IgG), as well as others. IgM is the first kind of antibody the body makes when fighting an infection. Sometimes IgM antibodies can be identified through a test when someone is still infected with a disease. IgG is the most common type of antibody and forms in the body over time after infection occurs.
Positive antibody tests are used to identify confirmed or probable cases for several communicable diseases. Depending on the disease, the presence of IgM or IgG can indicate past or current infection. For example, the presence of IgG antibodies, identified through an Enzyme Immunoassay (EIA), meets the laboratory criteria for a confirmed case of Coccidioidomycosis (Valley Fever). Other diseases, like many mosquito-borne diseases, tick-borne diseases, viral hemorrhagic fevers, hepatitis, HIV, and mumps are considered to be confirmed or probable infections in people with positive antibody testing.
Recently, antibody testing for COVID-19 has become more readily available in Arizona. At this time, it is unknown if a positive antibody test for COVID-19 means that someone is immune to COVID-19 or how long that immunity might last. However, a positive antibody test likely indicates a person was infected with the virus. Since COVID-19 has only been around since the end of 2019, we can assume this infection occurred very recently. It is important to note that antibody tests differ in their performance, so some tests may provide a false positive result, indicating that someone was recently infected with a coronavirus, but not specifically COVID-19.
When paired with an exposure to a COVID-19 positive case or symptoms consistent with COVID-19, such as shortness of breath, cough, and sore throat, a positive serology can identify a COVID-19 case that is counted in the total number of reported cases. This is consistent with the national recommendations for COVID-19 surveillance and reporting introduced by the Council for State and Territorial Epidemiologists (CSTE) and adopted by CDC.
It’s important to note that classification of cases is a surveillance tool used to better understand the burden of disease in the community, but does not necessarily impact the way an individual is treated for a disease by their healthcare provider. The use of current tests will evolve as we increase our understanding of COVID-19 and newer tests, such as antigen tests, become available.
Our Data Dashboard contains information about both diagnostic PCR and serologic antibody testing with options to filter the data by either type of test (see the blog post on these dashboard features). The data is displayed on the dashboard in three ways: the number and percent positivity of PCR tests, the number and percent positivity of serology tests, and the number and percent positivity of PCR combined with serology tests. Showing the total number of all tests performed and tracking this change over time demonstrates the increasing availability of testing for COVID-19 in Arizona. The number of tests and percent of positive tests are just some of the multiple indicators that we are using to track the spread of the disease and develop public health recommendations that will help keep our communities safe and healthy.
As we continue to get more data about COVID-19 in Arizona, we will look for new ways to display it on our dashboards so the public can better understand the status of the disease in Arizona. The data is updated seven days a week at 9 a.m. Please check back often for these data updates.