Warren County passed another milestone in COVID-19 cases in mid-August, exceeding 200 total cases detected since the start of the pandemic.

Over 270 county residents have tested positive for COVID-19 as of Tuesday, according to the Warren County Health Department, with 168 recovered. That leaves almost 100 active cases identified in the community.

Throughout the pandemic, public health officials have focused on providing information that can help limit the spread of the virus.

“Avoid large gatherings. Barbecues and weddings are where we have found the transmission to be the greatest,” said Warren County Public Health Administrator Ruth Walters.

That advice, along with wearing a mask in public, social distancing and staying home when sick, provide some clear guidelines to combating COVID-19.

How accurate are tests?

But one reoccurring question in the community is over how severe the problem really is, with some questioning the reliability of COVID testing and the accuracy of the local case count.

The underlying question: How often do COVID-19 tests return false positive results — telling someone they have the disease when they really don’t — and how often do tests return false negatives, missing the presence of COVID-19?

Although stories may be swapped over social media of people testing positive one day and negative the next, Dr. Emily Schindler, a medical director for Mercy St. Louis, said false positive tests are very rare.

“False positives are less than 1 percent,” and are most likely the result of human error rather than a problem with the test, Schindler said.

False negative tests are much more likely, she said, as high as 30 percent of negative results. That can be influenced by how long after being infected a patient is tested, and whether the virus is present in the area of the body sampled for testing, Schindler explained.

“It seems to vary significantly from person to person,” she said. “But it’s reasonable to go be tested if you had a known exposure.”

Schindler said the most important number to be aware of is what proportion of COVID tests are coming back positive. But a rising number of total cases in a community is still cause for concern.

“That plays into local transmission rates. The more people you have wandering around who have tested positive, the more possible it is for people to infect each other,” Schindler said. “You need to be mindful of all these little nuances (in data). COVID is still out in our community, and we need to be mindful of our precautions.”

Where tests can go wrong

Because this virus, and the tests that look for it, are so new, experts say they’re still learning about what provides the most accurate test results.

Any diagnostic test has the possibility of returning incorrect results, according to Johns Hopkins University of Medicine, one of the leading data institutes for COVID information. What matters is understanding and correcting for those errors.

For COVID testing, false negatives can occur if a sample is not properly collected, if it’s collected too soon after infection, or if there’s an error at the testing laboratory, according to Johns Hopkins. But those issues less often cause false positive results.

The accuracy of a test also depends on what kind of test it is. Broadly speaking, there are two kinds of tests that check for an active COVID infection.

PCR testing is a lab-based process that looks for the genetic material of COVID-19. It’s a slower process, but generally considered more reliable, according to Harvard Medical School.

Antigen testing looks for a particular protein present in COVID-19. The testing is less complicated and produces faster results, but has a higher chance of false negatives, according to Harvard Medical.

There’s also antibody testing, which looks for your body’s immune response. That can tell you if you’ve previously had the virus, but not when, or whether the infection is still active.

Harvard Medical says COVID tests, especially antigen tests, become more accurate several days after exposure to the virus, when more of the viral cells are present in a person’s body.

“All of these tests are new because the virus is new. Without a long track record, assessments of accuracy can only be approximate,” wrote Dr. Robert Shmerling of Harvard Medical in an Aug. 10 report.

And according to the Centers for Disease Control, because false negative results are much more likely, people and doctors should be careful how they interpret those results.

“Negative results should always be interpreted in the context of the exposure history and symptoms of the patient,” according to the CDC.