Testing for the novel coronavirus has become exponentially more accessible since the pandemic began. In Virginia, 43 times more tests were performed during the most recent week than in mid-March. Getting tested before traveling or returning to work has become a routine part of life during the pandemic; some universities are requiring proof of a negative test result from students before they return to campus this fall.
As of Monday, more than 1 million tests for the virus that causes COVID-19 have been conducted in Virginia since the pandemic began, according to the Virginia Department of Health, including 19,350 in the health district that includes Fauquier County.
However, the health director for the Rapidan-Rappahannock Health District and a local physician who oversees a testing program in Warrenton both cautioned that depending on several factors, diagnostic testing – tests that look for the active presence of the virus in an individual - has the potential to miss some infections. That is, a negative test result does not necessarily mean an individual is not infected with the virus.
“For any test to be 100% sensitive is not very common,” said Dr. Wade Kartchner, the health director for the Rappahannock-Rapidan Health District, which includes Fauquier County, “and when sensitivity of a test falls below 100%, this means that some negative tests are actually cases.” He said that false negatives are sometimes the result of human error, such as swabbing a patient incorrectly. Much of the variability, though, comes from when in relation to the onset of symptoms the test was conducted.
“Research shows that the likeliest time for a test to be positive when a patient actually has a disease is three days after symptoms start,” Kartchner said, elaborating later, “The sensitivity is not great prior to symptom onset, improves to 80% to 90% at around the third day after onset of symptoms, and then starts to decrease again after that.” He explained that, as the infection progresses from the third to the eighth day, the chances that a swab will retrieve enough of the virus’ signature to detect it are good.
Dr. Steven von Elten, a physician at Piedmont Family Practice who administers the coronavirus testing program at Piedmont Urgent Care, expanded on Kartchner’s assessment.
Three days before the onset of symptoms, he said, it is highly unlikely a test will detect the presence of the virus, even if an individual has an active infection. Two days before the onset of symptoms, diagnostic testing only detects about 20% of active COVID-19 infections. (Because the virus emerged so recently and large-scale peer-reviewed studies are still relatively few, there is little consensus about the exact detection rate.)
The sensitivity rate increases substantially as an individual begins to experience symptoms, von Elten said. If a person is swabbed during the first day, they are experiencing symptoms, the test detects about 60% of cases. By three to eight days after the onset of symptoms, the sensitivity rate rises to 80% or more. “If you’re symptomatic, you still have to stay home and isolate yourself,” von Elten recommended. “No test is 100% reliable.”
The swabbing method itself, von Elten explained, is also key in obtaining results that are as accurate as possible. The nasopharyngeal method used at his clinic is the most reliable method that is practical in a clinical setting, he said.
von Elten said he worries about people who have tested negative getting “cavalier” and not taking precautions to prevent infecting others unknowingly. “Please, please, please follow the [Centers for Disease Control] guidelines,” he said. (The CDC recommends that symptomatic individuals who test negative “should keep monitoring symptoms and seek medical advice about staying home and if you need to get tested again.”)
Kartchner echoed this sentiment. “[I]f one is sick and has a negative PCR [test], the public health recommendation would still be to stay home until symptom-free for 72 hours.”
Both Kartchner and von Elten said that false positives – when a person tests positive for an infection when they do not actually have that infection present in their body – from diagnostic coronavirus testing are far less common and far less of a problem than false negatives, but are not unheard of.
“Since the PCR test uses unique genetic sequences of the SARS-CoV2 virus, the specificity can approach 100%. This means that if one tests positive, they can reliably bank on having the illness. The greater issue is the sensitivity, since it relies on timing of the specimen and the technique used,” said Kartchner, adding that false-positive results can still arise from “cross-reactivity” with another strain of coronavirus or from simple human error, like mixing up samples.
“Rapid” antigen tests, Kartchner said, are typically less sensitive than PCR testing but are no more likely to produce a false positive. He said that the recommendation for individuals whose antigen test comes back negative but who are symptomatic is to be tested again with a PCR test. (Alaska, for instance, requires proof of a negative PCR test in order to travel to the state; an antigen test does not meet the requirements.)
It is less clear how much value antibody testing -- which measures the body’s response to a past infection and does not look for the infection itself -- is to an individual or to epidemiologists.
Kartchner explained, for instance, that false negatives from this type of testing can occur because not enough time has passed after an infection for an individual to develop antibodies. False positives can occur because the antibodies developed in response to other coronaviruses are very similar to those developed in response to the coronavirus discovered in 2019.
“A positive test result shows you may have antibodies from an infection with the virus that causes COVID-19,” says the CDC website about antibody testing. “However, there is a chance a positive result means that you have antibodies from an infection with a virus from the same family of viruses (called coronaviruses), such as the one that causes the common cold.”
In any case, there is no understanding so far as to how much – if any – immunity to future infections from the novel coronavirus antibodies provide, Kartchner said.