Russ Daly: COVID-19 tests are two different animals

Russ Daly
Special to the Farm Forum
Russ Daly

For veterinarians, lab testing is an everyday necessity.

From morning to night we’re presented with ill animals for which we need information beyond our hands-on evaluation. We might want to know the level of a certain blood enzyme that could tell us if a cat has liver disease, or whether a dog’s red blood cell count tells us they’re anemic.

Finding germs is also a frequent reason for lab testing in animals – especially in livestock. Bacteria and viruses are at the root of animal diseases much more frequently than they are in human medicine, it seems.

The COVID-19 pandemic has changed that somewhat. Getting a COVID test has become a daily conversation topic, and detecting the virus is a prime focus of human health care.

Back at the beginning of the pandemic, there was only one type of COVID test available. Your nose or throat swab was shipped to a laboratory that performed a PCR, or polymerase chain, reaction test. It usually took a while to get results, but those results were pretty solid.

We still extensively depend on PCR COVID tests, but now other test types are available, many of which you can buy and use yourself.

Having more types of COVID tests available is great, but it does add some confusion. Adding to the uncertainty are all the different terms used in describing these tests. Viral tests, antigen tests, laboratory tests, rapid tests, home tests, diagnostic tests. Wow. How can anyone sort all these out?

Let’s examine those two broad categories of COVID “viral” tests: those that detect the actual virus. PCR tests, also known as “nucleic acid amplifying tests,” use the more complicated technology. We use these tests every day here at South Dakota State to detect germs in animal samples. Because they exponentially magnify the bit of genetic material specific only to that germ, they can detect extremely low levels of virus – even if it’s not viable anymore – making them very sensitive and specific.

The downside of a PRC is that it requires special lab materials and machinery more expensive than a new F-250. This type of test therefore needs to be run in a lab with the right equipment and expertise.

The other category includes the “rapid” or “antigen” tests, those used in your home or doctor’s office. Your nose swab is placed in liquid, which you then drop onto a paper strip or cartridge. Invisibly incorporated into that paper is a stripe of antibodies against the COVID-19 virus. If your sample contains the virus, the virus sticks to the antibody stripe on the paper and creates a color change. No expensive equipment needed.

The thing is, these rapid tests need a lot more virus in the sample to pop positive compared with PCR. Understanding your result, therefore means we need to know which type of test was used along with how the COVID-19 virus acts in the infected person’s body.

Think of the amount of virus in an infected person over time as something we can plot on a bell curve. When a person first encounters an infection, virus levels are very low. As the virus expands in the body (coinciding with any signs of illness), the curve goes up and peaks. As the body’s immune system cleans up the infection, the curve drops back down.

PCR tests are so sensitive they can detect virus almost anywhere along that curve: earlier than a person becomes ill and after they’ve recovered and are no longer able to infect others.

Since rapid, or antigen, tests need more virus to show positive, they primarily detect the virus during the higher, middle part of that bell curve, right before or during illness – and when a person is most likely to be transmissible. As such, they’re less likely to detect early infections but are also less likely to stay positive after a person is no longer infectious.

Which test is best? For most situations, the rapid tests are just fine. It’s possible a rapid test could be negative while a person is still infectious, but that’s not the majority of cases. The high sensitivity of a PCR test means you could be positive for a while yet not infectious; that’s why the CDC puts more stock in how long it’s been since you’ve felt sick rather than a PCR result to indicate if you’re infectious.

Here’s hoping you’re staying healthy through this latest stage of the pandemic, and that you don’t encounter a situation where you need a COVID test. But if you do, knowing a little about your test will help you understand what your result really means.

Russ Daly is the Extension veterinarian at South Dakota State University.  He can be reached via e-mail at russell.daly@sdstate.edu or at 605-688-5171.