In fact, however, the re-opening effort is in danger of failing spectacularly because the U.S. hasn’t gotten the first and last steps right — which both rely on accurate diagnostic testing. The current swab tests for the novel coronavirus are missing up to 30% of infected people.
In a scramble to fix that before it’s too late, a growing number of doctors are calling for use of another method to detect Covid-19, one that would miss fewer cases than molecular testing of swabs does: chest CT scans.
CT scans are far more expensive, they expose patients to a low dose of radiation, and the Centers for Disease Control and Prevention and some medical groups recommend against using them to diagnose Covid-19. But they were widely used in China to identify cases, and their reliability there is fueling growing interest in adding chest CT to the diagnostic arsenal in the U.S.
The scans detect hazy, patchy, “ground glass” white spots in the lung, a telltale sign of Covid-19. In one recent study of 1,014 patients, published in the journal Radiology, scientists in China reported that chest CT found 97% of Covid-19 infections. In comparison, the study found that 48% of patients who had negative results on the swab test, which detect the coronavirus’s genome, in fact had the disease.
“Once you’re a couple of days into infection, chest CT scans don’t miss,” said an emergency medicine physician in Louisiana who asked not to named. With the swab test missing 30% to 50% of cases, physicians in China called for the diagnostic use of CT early in the outbreak there, and “fever clinics” set up in Wuhan and elsewhere began routinely using them.
A positive result on the swab tests is usually reliable. “If you get a positive test result, looking for the RNA of the virus with the current methods that we have, it’s very likely to be a true positive,” said Jana Broadhurst, an infectious disease doctor and director of the Nebraska Biocontainment Unit Clinical Laboratory at the University of Nebraska Medical Center. But “if you get a negative test result, [the chance that it’s wrong is] about 30%.” Of every 100 symptomatic people who test negative for Covid-19, 30 are actually infected. The test missed them.
The main reason is sample collection. The swab that’s supposed to be pushed into the back of the nose (often painfully) and then curve down into the throat sometimes doesn’t reach far enough, or doesn’t remain in place long enough, to collect a decent sample. The swab can trigger violent coughing, making the technician or nurse taking the sample pull back too soon. “We have a mantra in the lab,” Broadhurst said: “Garbage in, garbage out.”
Because the fault is human rather than molecular, there is no technological fix. If tests are the first leg of an exit strategy, as the Johns Hopkins Center for Health Security said in a plan released last weekend for reopening the U.S. economy, then incorrectly “clearing” 30% of those who are tested will doom any exit plan. They could be cleared to return to work when they’re actually infectious, and — even worse — those they encounter and potentially expose to the virus would not be identified and quarantined.
“We cannot rely fully on the test” to guide decisions crucial to re-opening the economy, said Sandro Galea, a physician and epidemiologist who is the dean of the Boston University School of Public Health.