There are two types of tests available for COVID-19 that can detect whether a person is or has in the past been infected with the SARS-CoV-2 virus (which causes COVID-19 illness):
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Polymerase chain reaction (PCR) testing, which tests for active infection; and
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Serology testing for antibodies against SARS-CoV-2.
Diagnostic PCR Testing
The most commonly used and reliable test for diagnosis of COVID-19 has been the RT-PCR test performed using nasopharyngeal swabs or other upper respiratory tract specimens, including throat swab or, more recently, saliva.
The PCR test uses a long nasal swab to collect a specimen in the nose, throat, or other areas in the respiratory tract to determine if there is an active infection with SARS-CoV-2. This test is useful for determining who has an active infection and identifies people who are contagious to others.
Antibody Testing
Serology testing uses either a drop of blood or a blood draw and is used to determine if a person has been an infection in the past. IgM is the first antibody that is formed against an antigen, so it appears on tests first, usually within 1-2 weeks. The body then forms IgG, which appears on tests about 2 weeks after the illness starts. IgM usually disappears from the blood within a few months, but IgG can last for years. Some antibody tests test for IgM and IgG, and some only test for IgG.
Serological diagnosis is especially important for patients with mild to moderate illness who may present late, beyond the first 2 weeks of illness onset. Serological diagnosis also is becoming an important tool to understand the extent of COVID-19 in the community and to identify individuals who are immune and potentially “protected” from becoming infected.
Using Test Results to Inform Public Health Interventions
The federal government has issued guidance for states to use as they make decisions about whether to tighten or loosen interventions like stay at home orders. Some of the criteria in that guidance includes the trajectory of documented cases (as measured by these tests).
For example, gating criteria number 2 states that states should document a: 1) downward trajectory of documented cases within a 14-day period; or 2) a downward trajectory of positive tests as a percent of total tests within a 14-day period (flat or increasing volume of tests) before moving to a “Phase I Opening”.
However, the Gating Criteria aren’t specific about whether “positive tests” are just the RT-PCR nasal swab test or whether the criteria can include antibody tests.
The percent positives for the RT-PCR tests and the serology tests are quite different (the percent positive for RT-PCR tests in AZ is currently 8.1% while the serology is 3.7%). That makes sense, because the PRC tests are conducted on persons that are symptomatic and mostly in clinical settings while the serology tests are usually done on asymptomatic folks. And remember, the serology test (if positive) suggests that the person was infected at some point in the past, which could be weeks or even months ago.
So, how should we interpret these tests for the purposes of determining whether we meet Gating Criteria Number 2?
The only thing that makes sense is to track the trajectory of the RT-PCR tests and Serology tests separately– and look at the trends separately. That’s because each of these tests are for different purposes and are used on different populations. Both are valid for their own intent, but they shouldn’t be put in the same data set.
We urge the ADHS and governor to track and interpret PCR and Serology tests separately… and keep the data sets separate when looking at data trends for the purposes of determining whether we meet the benchmarks in the federal guidance. The ADHS data dashboard is now tracking these data sets separately – so that’s a good thing.
Hopefully that stands for decision-making purposes for determining when Arizona is ready for Phase I under the federal guidance.
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Related Journal Article of the Week
Interpreting Diagnostic Tests for SARS -CoV-2
JAMA. Published online May 6, 2020. doi:10.1001/jama.2020.8259
