Test again? Yes!
The University of Idaho is offering testing on an ongoing basis to students, faculty and staff. So you might be wondering whether you should get tested again, and when you should do it.
If you traveled for Labor Day weekend, or had contacts with individuals outside of your usual network, you should seek one of these tests. While every test has some chance of a false negative reading, you can minimize the chance of having a negative test result by waiting until the latency period for the virus has passed and there is sufficient time for virus in the nasal passages to be detected. How long? About five to six days. For those of us who had an exciting contact-filled Labor Day weekend, now is the time!
What is a false negative? A false negative occurs when a clinical test does not indicate the presence of the condition that is suspected, but that condition is actually present. Timing can matter a lot in how likely you are to get a false negative. For example, a pregnant woman is very likely to have a negative pregnancy test if she takes it the first day she is late for her cycle. However, a negative test a full week later would be very unlikely (~0.1% probability according to the packaging). This probability of having a positive test when the condition is present is called the sensitivity of the test.
What about false positives? False positives are tests that give a positive reading when the condition that is suspected is not present. The advantage of the genetic PCR test for COVID-19 is that the false positive rate is very low, generally less that 4%. Let’s break this down a little bit. If 4% of people who are negative for COVID-19 test positive that would mean that 1 in 25 people would be testing positive when they don’t have the virus. That’s a lot! However, if we suppose that each test is independent, then the probability that both tests are false positives goes all the way down to 0.16%, or 1 in every 625 people. While this is much more palatable, it gets even better if we assume that the true positive rate or specificity of the test is 99%. In this case, the chance that someone who does not have COVID-19 will test positive twice is 1 in 10,000. Here’s a video on test validity, if you want to learn more.
Why is traveling a big deal during the pandemic? The best way to understand this is to think about the fires that are in the West right now. They are fanned by high winds, but fed by dry forests and underbrush. That tinder can act as an accelerant, driving a few stray embers into a deadly inferno. Physical contacts are the underbrush of disease spread. When we travel, we are creating pathways for the virus to spread between groups who might not otherwise be exposed to one another. By practicing social distancing from your close contacts on your return and getting tested after the latency period, you are ensuring that you are not carrying embers of disease back into our university community.
To all of you who can describe in great detail the eye-watering discomfort of the sinus swab, we all thank you!
Jennifer Johnson-Leung is a mathematics professor who uses models to explain and understand complex systems and to bring abstract concepts into focus for a wide audience.