SARS-CoV-2 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 in Virginia. This method of looking at testing measures will only count a person once regardless of how many times they are tested over weeks or months. 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 our 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 high-risk, and some are known cases who need to have negative tests to return to their normal life. VDH started reporting this method on May 1 as a better way to measure Virginia’s capacity to test people. It is included in the daily COVID-19 Cases in Virginia dashboard, the dashboard of cases and tests by ZIP code, and the Key Measures report. Regardless of how many times a person has been tested, they will only be counted as a case once.
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 that are conducted on people who are sick enough to pursue testing while they’re 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, so VDH is not counting someone as a case with that information alone.
- 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 for 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 FDA-approved and some are not. The FDA has issued an Emergency Use Authorization, or EUA, for a lot of tests when the manufacturer has been able to show good sensitivity and specificity. This is not as rigorous a process as new tests normally need to go through to be approved. There are some tests that are not FDA-approved at all being used as well. VDH is only reporting FDA-approved tests.