COVID-19 testing data are complex. There are several ways of looking at these numbers, and VDH has used several of these methods over the course of our COVID-19 response.
- Unique people tested
The number of unique people tested is how we look at the overall number of individuals who have been tested for SARS-CoV-2, the virus that causes COVID-19, in Virginia. This method of testing will only count a person once regardless of how many times they are tested. VDH used this method originally as a way to measure the number of opportunities to identify a new case.
- Testing encounters or Total people tested
VDH has referred to this measure by two names – total people tested and testing encounters. Both of these names were an attempt to communicate that this is the number of people who have been tested per day. Over the course of the COVID-19 response, some people have been tested more than once. Some of these people are healthcare workers, some are at higher risk, and some are known cases who need to have a negative test to return to work or other activities. VDH started reporting this method on May 1, 2020 as a better way to measure Virginia’s capacity to test people. It is included in the daily COVID-19 Testing dashboard and the COVID-19 Case and Testing Data by ZIP code dashboard
Besides these two methods of measuring the number of tests Virginia has conducted, there are a few other things to keep in mind.
- VDH is reporting test numbers that include people who do not live in Virginia. If we want to measure the state’s capacity to test for SARS-CoV-2, we need to include all tests of people who are sick enough to seek out testing while they ar in Virginia.
- Case data and test data are two different sources. Not all cases involve a positive test, and not all positive tests count as cases. Some cases are counted based on their clinical symptoms alone, so those people are not included in testing data. Some tests are in out-of-state residents, so those people would not be included in Virginia’s case numbers. Other tests are positive for antibodies or an antigen. These tests are not as accurate as RT-PCR, the gold standard.
- Not all tests are equal. Sensitivity is the measure of how likely a test is to identify an infection. Specificity is how likely a positive result is to be due to the exact pathogen the test is designed for. The higher the sensitivity of a test, the lower the rate of false negatives. The higher the specificity, the lower the rate of false positives. RT-PCR tests look for the virus’ genome and are both highly sensitive and highly specific. Other tests, like antibody tests, are less accurate. These tests look for the antibodies that our immune system builds after infection with a new pathogen. These antibodies take a few days or weeks to form, so a test conducted too early may not have good sensitivity. These antibodies often look similar for related pathogens. Because there are several regularly-occurring coronaviruses in the human population, some public health officials are worried about cross-reactivity. Right now, antibody results may need to be confirmed using an RT- PCR test.
- Some tests are approved or authorized by the Food and Drug Administration (FDA) and others are not. The FDA has issued an Emergency Use Authorization (EUA) for a lot of tests when the manufacturer has been able to show good sensitivity and specificity. VDH is only reporting FDA-approved or FDA-authorized tests.
*Originally posted on May 7, 2020.