Dr. Amy Fan Conrad
How to know which COVID test is right for your family?
Needing a COVID test has become one of the many strange and universal experiences of 2020. But choosing a test can be a test in itself!
What are the differences between the tests, and when is each one appropriate? Let’s do an overview of these tests and how to think about which one we need.
First, there are two large categories of tests - diagnostic vs. serum antibody.
Serum antibody tests answer the question of “have I already had COVID-19?” Instead of testing for the presence of the virus in the body, this examines whether the body has already encountered the virus in the past, and created antibodies in response, which then linger in the body.
Format: blood test
Timing: after COVID symptoms have passed. Would not be expected to be positive during the initial infection, or even immediately after.
Use: to identify those who already have natural immunity to COVID from having had an infection, perhaps asymptomatically, in the past. This can be useful to help decide who receives the first batch of vaccines and other decision making policies.
Diagnostic tests answer the question of “do I have COVID-19 right now?” This type of test includes both the oral and nasal swabs that everyone is familiar with. It is further divided into two testing methods - PCR vs. rapid antigen.
First, I want to introduce the concept of “sensitivity and specificity” of tests. These are technical terms in the study of epidemiology, and describe a test’s ability to have a reliable result. We don’t have to get into the math, but the take-away is:
Sensitivity - the ability of a test to detect a condition when it, in fact, exists. High sensitivity means there are fewer false negatives. PCR tests are high in sensitivity.
Specificity - the ability of a test to show a negative result when the condition, in fact, doesn’t exist. High specificity means there are fewer false positives. Rapid antigen tests are high in specificity.
It can feel like a tongue twister for the brain! In real life practical terms, however, it means that we usually want a sensitive test as a screen test (to capture as many positives as possible), then a specific test to confirm (to identify as many negatives as possible).
So what does this mean for anyone trying to pick the right test?
A PCR test is more effective as a screening test, especially for asymptomatic people. It’s much less likely to have a falsely negative result. On the other hand, the rapid antigen test should only be used in a clinical setting to confirm or monitor COVID-19 infection. The best time to get a PCR test is 5-7 days after exposure, as it takes time for the virus to replicate enough to achieve a detectable level.
Nasal vs saliva tests should be equivalent in this case, as long as they both lead to a PCR result from an FDA-authorized testing center.
I hope that this post has been helpful navigating all the various types of COVID tests out there and which test is best suited for your family. Kinder is now offering at-home PCR testing for our members, with an easy work flow including triage by your own personal pediatrician. Find out more or talk to a Kinder pediatrician here.