St. Luke's Coronavirus Testing

Drive-thru coronavirus testing, pictured here last month, is still available outside St. Luke's Wood River Medical Center in Ketchum. 

As testing becomes more streamlined, with new equipment allowing for faster turnaround and large quantities of samples being tested on a daily basis, questions remain about the accuracy of COVID-19 testing and what questions antibody testing can answer.

Boise-based infectious disease expert Dr. Sky Blue spoke with the Idaho Mountain Express regarding false negatives in COVID-19 testing and about antibody testing making its way into the market and what it can tell us.

Currently, the most common type of COVID-19 testing is PCR—polymerase chain reaction—testing, which tests samples collected from the nose or throat to look for RNA—ribonucleic acid—from the virus. A false negative—that is, a sample that tests negative for the virus when the person is in fact positive for COVID-19—can occur in two scenarios. First, a false negative result can occur if a sample is collected from someone who is past the peak of his or her infection or in the infancy stage of infection, meaning there is less RNA available to test. Though PCR tests can detect very small amounts of RNA, which is amplified through a cycling process on a laboratory machine, most labs have a threshold for how many cycles a sample will go through to detect RNA.

Second, if a swab collected doesn’t have enough secretion in the sample, the PCR test may not detect sufficient RNA to find a test positive, though it could have been that there was just no secretion in the nose when the sample was collected.

In general, Blue said, the PCR test results are dependent on when the swab is collected; the better a patient feels, the higher the chance that the sample may result in a false negative because there is less of the virus in the person’s system.

Antibody testing

According to Blue, there are two types of antibody testing: lateral flow assay and ELISA—enzyme-linked immunosorbent assay. The first is a rapid test that uses a small amount of blood and is most accurate about seven days after the onset of symptoms, because early in the infection antibodies might not yet be present. However, like other rapid tests, such as pregnancy and strep throat tests, it usually requires a more thorough test, such as a blood draw, to confirm the results.

Lateral flow assays may also react to other viruses, such as those found in a common cold or influenza and may produce a positive finding for antibodies created to fight either of those viruses rather than the COVID-19 virus.

According to Blue, the more accurate antibody test is the ELISA, because it requires a large sampling of blood and a more thorough process, spinning down the blood sample with an antigen to see if they bind.

This test can also be inaccurate if the blood sample is taken too early in the infection, when antibodies may not yet be present, Blue said.

The benefit of antibody testing is being able to see how many people in the community have had the virus, but it is still not known if the test will be able to say if someone is immune and if so, for how long.

Though some testing on monkeys indicates immunity after being infected with COVID-19, it is unknown how long that immunity will last, Blue said.

The duration is different for different diseases. Someone who is vaccinated for hepatitis B has immunity for the rest of his or her life. But vaccinations for influenza should be administered annually. According to the Centers for Disease Control and Prevention, that’s required both to cover mutations in the virus and because a person’s immune protection from vaccination declines over time.

The details of the antibody study and testing coming to Blaine County are still opaque. Ketchum Assistant City Administrator Lisa Enourato said more details will be shared about the study and testing by Thursday, April 16.

The curve

“There are so many variables,” Blue said when it comes to determining if in fact the curve of infection is flattening. “It’s so hard to know.”

Locally, few new cases today may indicate a decline, but that only remains true if things stay as-is—meaning no one travels in or out of the county and people maintain the same level of social distancing. Any change to any variables may change the curve and any influx of visitors may bring more cases, causing another spike. Though there may be a point of herd immunity, where that point is remains unknown, Blue said.

In addition, there may be more immunity in certain age groups than others, creating yet another unknown variable of what could occur if a younger population begins interacting with a more vulnerable older population that might not have full immunity.

According to Blue, it is unknown if Blaine County has seen the curve flattened due to the measures put in place or because the it has naturally gone through its apex.

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